“Stop fidgeting, Leela,” The Doctor said. Leela scowled at him and adjusted the waistband of the knee length skirt she was wearing with a neat cotton blouse and sandals that tapped noisily on the pavement. “You look very demure.” Early twenty-first century London had probably seen worse than her chamois-leather dresses. After all it had seen out the mini-skirts of the nineteen sixties and the punk era of the nineteen seventies and much that was worse. But The Doctor thought Leela’s outfit today was perfectly suitable for the occasion.

“Do I want to be demure?” she asked. “What does it mean, anyway?”

“It means you are a respectable young woman,” The Doctor answered. “Please try to behave as if you are.”

He paused outside a tall seventeenth century town house that had been converted into offices and then marched right into the foyer. He checked the list of businesses and headed towards the ground floor offices of an old established firm of solicitors.

“Good afternoon,” he said to the receptionist. “I am The Doctor. I understand you are holding a document for me.”

“Do you have any photo identification?” the receptionist asked.

“Yes, I do. Just one moment,” The Doctor replied. He reached into his voluminous pockets and pulled out an assortment of odd contents. A yoyo and a bag of jelly babies came first. The receptionist politely refused the offer of a sweet and watched as he deposited the sonic screwdriver, a ball of string, a collection of coins, none of which were legal tender on planet Earth, before he found a small plastic wallet containing various forms of identification. “My U.N.I.T pass,” he said. “Ministry of Defence stamped.”

The photograph on the pass was actually of his third incarnation. But he smiled widely at the receptionist and his hypnotic eyes bore into hers. She gave the ID card back to him and asked him to wait a few minutes. He put his collection of personal possessions back into his pocket before a young man in a smart suit came out of a door and introduced himself as Mr Gordon Orpington, LL.B.

“You’re The Doctor?” he asked.

“I am,” The Doctor replied.

“You saw the advertisement in The Times requesting your attention?”

“I did. And the young lady has checked my identification thoroughly.”

“Yes, of course. Won’t you come this way?” Mr Gordon Orpington led The Doctor and Leela through to his well appointed office and offered them both seats. He sat down on his own chair behind the highly polished desk and reached in a drawer for a small package covered in very old oilskin. He handed it to The Doctor reverently.

“It has been in the possession of Orpington, Orpington and Pine since the partnership was established by my great grandfather in 1895,” he said. “It was discovered in a cavity within the wall of this very office when the building was being renovated. As you can see, the instructions on the front of the envelope are clear. A message placed in The Times on the first Monday of every month, until The Doctor should make contact. The instruction was carried out.... for one hundred and fifteen years.”

“That was very conscientious of you and your predecessors,” The Doctor said. “Well done. I’m sorry I didn’t check the personal ads much sooner than this. I should be happy to reimburse your firm for the cost of the placements...”

“Er... I... really don’t think that matters,” Gordon Orpington replied. “It would be difficult to calculate the full cost at this stage, in any case. This property dates back to the seventeenth century, of course, when Buckingham Street was built on the site of the former York Palace. The date beneath the instructions is February, 1683.... when this very building was occupied by...”

“Yes, indeed,” The Doctor said as he broke the wax seal and opened the envelope preserved within the protective oilskin. Gordon Orpington looked at the same time appalled that something his firm had protected carefully for so very long had been so wantonly desecrated, and fascinated to know what was inside the envelope.

“Well, well,” The Doctor said after perusing the hand written letter carefully. “Well, I never... That really is interesting.”

Gordon Orpington almost fell off his chair in frustration. He really wanted to know the secret his family’s firm had been keeping all these years. But it didn’t look as if The Doctor was going to let him in on it.

“Tell you what, Gordon” The Doctor said at last, putting the letter back into the envelope and slipping it into the oilskin. “If you take this to a reputable auction house that deals with historical ephemera, you will more than recoup your expenses. It is a well known historical fact that the author of this missive rarely put pen to paper himself after 1669 due to failing eyesight. A letter personally penned by him in 1683 is quite a rarity.”

The Doctor stood. Leela did the same, following him out of the office. Gordon Orpington waited thirty seconds before sliding the envelope out of the oilskin and taking out the letter from the envelope.

“My dear Doctor,

It is to be hoped that this finds you in good health. I am in goodly manner myself at present, though I shall be glad to see the end of these cold winter months.

There have been disturbing events in this neighbourhood of late. Deaths which cannot be accounted for by any natural cause and much gossip among the lesser classes on the manner of Ungodly Things. Might I beg your attendance. It is possible that a mind such as yours might penetrate the gloom of these uncertain days.

Your friend,

Samuel.

This day, February 14th, in the Year of Our Lord, 1683.”

“Stop fidgeting, Leela,” The Doctor said. Leela scowled at him and adjusted the lace cuffs of her dress, almost losing hold of her matching parasol in the process. “You look very elegant,” he added appreciating the tight bodice and wide overskirt of deep blue velvet and the yellow satin and lace underskirt that he had shown her how to lift over the drifts of snow lying on the wintry ground. It was considerably more clothing than Leela customarily wore. He tried not to imagine what the City of London in 1683 would have made of her usual thigh length chamois-leather outfits.

“Do I want to be elegant?” she asked. “What does that mean?”

“It means you are a respectable young woman,” The Doctor replied. “This is the seventeenth century, after all. And we are going to visit a respectable gentleman of the time.” The Doctor lapsed into thoughtfulness for a moment. “Actually, this respectable gentleman could be a bit naughty. If he does attempt any... inappropriate actions... please just refuse his attentions politely. Don’t try to break his limbs or render him unconscious in any way. Young women of this time are not usually skilled in methods of unarmed combat.”

“How will I know what is inappropriate?” Leela asked.

“You’ll know.” He stopped in front of a well appointed London town house and looked up at the curtained bay windows of the drawing room. There was a warm lamp glow from there. The master of the house was home. Since it was three o’clock on a February afternoon and the night would soon be drawing in that was a reassuring sight.

He mounted the steps and rang the bell at the side of the wide door. It was answered presently by a young woman dressed as a maid.

“Good afternoon,” The Doctor said. “Could you be so good as to announce The Doctor and Mistress Leela to your master?”

“Is he expecting you?” the maid asked.

“Just tell him The Doctor is here,” The Doctor replied.

“You’d best step into the hall, sir, ma’am,” the girl said nervously before scurrying off towards the drawing room. She returned presently to show her master’s guests into his presence.

“Doctor! My old friend,” said the gentleman sitting by the fireplace in a warm rest gown. He struggled to his feet to greet them properly. He was in his fifties, and showing signs of intermittent illness, including a yellow tinge to his complexion that suggested a kidney complaint. He squinted a little in the lamplight, too, but he stood straight and steady as he shook hands with The Doctor and bowed politely to Leela before taking her hand and kissing it.

“Is that inappropriate?” Leela asked The Doctor uncertainly.

“It’s a little forward since I haven’t yet introduced you,” The Doctor said with a laugh.

“Then introduce us,” the gentleman said in response. “Do not keep such a fair maiden’s name secret.”

“Samuel, this young lady is my good friend Leela of the Sevateem. Leela, this is my old friend Mr Samuel Pepys of London.”

Samuel Pepys bowed again and attempted to kiss her hand a second time, but she drew both hands behind her back.

“Come and sit,” he said to them both. “Bring the chairs closer to the fire. It’s a bitter night. We are likely to have fresh falls of snow, yet.”

“I see you are hard by the river in this house,” The Doctor said. “You have moved since I saw you last, even though your house in Seething Lane survived the Great Fire.”

“Needs must,” Pepys replied. “Since my dear wife died, I have found several places to lay my head. A few years ago, as you must surely know, I was compelled to lay it upon a most uncomfortable pillow in a cell of the Tower, itself. The charges were dropped forthwith, and I am in favour again in the circles that matter most. But this modest place, rented from a good friend, serves me well enough.”

“It is certainly better than your lodgings in the Tower,” The Doctor agreed. “I remember bringing some comforts to Sir Walter Raleigh when he was thus confined....”

“Sir Walter Raleigh!” Pepys laughed merrily. “Doctor, your tall tales get taller every time we meet.” He rang the bell to have refreshments brought. These turned out to be a flagon of wine that Pepys warmed and spiced over the fire and cheese and bread. The Doctor tried to show Leela how to eat the food according to late seventeenth etiquette, but in the end she speared the bread and cheese with a knife and ate her fill, washing it down with a gulp of the heated wine.

“What is the trouble you’re having?” The Doctor asked. “In your letter, you spoke of unnatural deaths...”

“Letter?” Pepys was puzzled. “But....My dear Doctor, you astound me. How could you possibly know the contents of an envelope I sealed only a few minutes before you arrived at my door? There it is on the bureaux. I wrote it myself, despite my failing sight. I did not want to dictate such an important and personal letter to a clerk. No-one knows what is written except my own self.”

“I know,” The Doctor said. “After all, the letter is addressed to me.”

Samuel was perplexed. But he had no opportunity to pursue the issue. An uproar outside in the street coincided with the maid rushing into the room without knocking.

“Sir,” she gasped. “Oh, sir, come quickly. There’s been another body found... right by the Watergate. Oh, it’s ghastly, they say.”

The Doctor stood, wrapping his scarf around his neck. Samuel stood, too, telling the girl to fetch both of their outdoor coats. Leela followed them as far as the front door before the master of the house turned to her in surprise.

“Good gracious. This is no work for a maiden.”

“I do not fear death,” she replied with such a steely expression that Samuel was disconcerted. He turned from her and rushed to catch up with The Doctor. He had already reached the elaborate Italianate archway at the end of Buckingham Street known as the York Watergate. When he and Leela had visited Orpington, Orpington and Pine in 2010, the gate lead to a landscaped garden some hundred metres away from the later course of the river. But in 1683, the steps led right down into the Thames. Directly in front of it was a mudbank where rowing boats were moored and on the other side of the main part of the river was a huddle of masts from anchored sailing ships.

The Doctor hurried down the steps to help the boatmen lift a body wrapped in oilskin from a rowing boat. He noted that it was peculiarly light before it was laid down on the top step of the Watergate and unwrapped.

“Good gracious!” Samuel exclaimed as he looked at the remains. He turned and again tried to persuade Leela that this was not for her eyes.

“He was one of the watermen?” The Doctor asked as he noted the remains of homespun trousers and a leather jerkin such as the two boatmen were wearing. The clothing was muddy and torn, but the body was in even worse condition. It looked half eaten. All of the flesh had been stripped away and the soft internal organs gone. The eyeballs were eaten away, too. Remnants of raw meat hung on the skeleton.

“Jem Robbins,” said one of the boatmen. “We found him lying in his boat... over yonder.” The man pointed to the mudbank. It was in deep shadow now with the sky darkening. The Watergate was lit by torches held by the servants from some of the other houses of Buckingham Street who had come running, but the river was dark. Even The Doctor, with his superior eyesight, had trouble making anything out.

“Doctor!” Leela called out in agitation. “Something moved... out there. Something white... it’s...”

There was a splash and the sound of agitated swimming. Somebody was crying and gasping and trying to reach the steps of the Watergate. The distance was not far. Ten yards of murky water lay between the mudbank and safety, but the struggling figure didn’t look as if it was going to make it.

There was a rustling sound and then, while the men stared, Leela, dressed in only a small satin underslip, leapt into the river. The Doctor glanced around and saw her velvet dress lying in a heap then rushed down the steps. He was ankle deep in the water as Leela reached the swimmer and began to pull back towards the shore. He called for more light and the torchbearers drew closer. The yellow, flickering light illuminated the surface of the water as The Doctor and Samuel both reached to help Leela and the swimmer to dry land. Then the torchlight wobbled. The bearers swore in a way that really would have offended a delicate lady’s sensibilities and ran back up the steps ahead of everyone else, away from the thing that had briefly surfaced close to them.

“Was that....” Samuel exclaimed as they reached the top step. “Was it....”

“Yes, it was,” The Doctor replied. “You men... wrap that body and bring it to Master Pepys house. Take it to the cellar for now.”

Samuel Pepys looked a little disconcerted by that idea at first, but The Doctor’s expression brooked no refusal.

“Yes, do that,” he said. “Then go to the kitchen. There’ll be a drink for your trouble, and a coin or two if you seek me out later.”

“You other men,” The Doctor said as they set to work. “Do you see that large blue box, standing on the pavement on the other side of the Watergate? Have that brought to Master Pepys cellar, too. A drink and coinage for your trouble. Leela... come along and bring that strange fish you caught.”

“It’s not a fish, it’s a boy,” she answered, failing to understand The Doctor’s humour.

“Take him to the drawing room,” Samuel ordered her. “It’s the best fire in the house. Dear me, this is a bad situation.” He bundled up Leela’s dress and brought it with him as the party headed back through the Watergate and into his house, the closest one to the river. There was much that disturbed him. A nearly naked woman and a ragamuffin boy, both dripping wet, going into his well appointed drawing room was the least of it.

The maid became busy with towels and blankets and, at her master’s instruction, the preparation of tea, a beverage more valuable than wine in a seventeenth century household. The young mudlark, wrapped in a blanket by the fireside, was surprised to be given a china cup with the hot liquid in it. He grasped the cup in both hands, dispensing with the saucer. Leela, sitting on a chair, similarly wrapped, did the same. The Doctor and Samuel adhered to the more usual tea-drinking etiquette before beginning to question the boy. He managed to tell them that his name was Tom Haggerty, that he was fourteen years old, and worked with the Thames boatmen.

“I was jus’ getting a bit of kip in the boat,” he said. “The shouting woke me. And I was watchin’... all the boatmen and the toffs and all, at the gate, pullin’ up the body... Then I saw... somethin’ swimmin’ in the river.... comin’ after me, it were. Oh, sirs, it were ‘orrible. Like a... a....”

“We know what it was like,” The Doctor told him. “We all saw it, too. Jumping into the water to get away from something that can obviously swim better than you might not have been the brightest of moves. But you’re safe now. Don’t you worry.”

“I ain’t never goin’ near the river again,” Tom vowed. “I’ll... I’ll run away to the country an’ plough the soil. Jus’... jus’ so I never look at a thing like that again.”

“That’s the spirit,” The Doctor said. “Can you tell me what direction the creature came from? Can you remember?”

“From Blackfriars way,” Tom managed to tell him. “Comin’ upriver agains’ the tide.”

“Good lad.”

“A very god lad, indeed,” Samuel agreed. “He’s done his duty. I think what he needs now is some clean, dry clothes and a shilling or two for his pocket. I’ve got the shillings and Mary-Anne can open up an old trunk in the lumber room where there’s some clothing to fit him. And when young Tom is fitted out, she can help Mistress Leela dress herself proprietarily.”

“An excellent notion,” The Doctor agreed. “Meanwhile, let us take a closer look at the body that was placed in your basement.”

Leela was not happy about being shunted off with the maid to put on elaborate clothing again, but The Doctor insisted on it. Tom was still making plans to put as many miles as possible between him and the Thames, and clean clothes and shillings would help start him on his way.

The body in the basement was a pitiful sight and one to upset the appetite of a man who had not yet had his evening meal, as Samuel Pepys observed. But it did not, on the face of things, yield any new clues.

“The body should be removed to the morgue as soon as possible,” Samuel suggested. “I’ll see to the fee for a burial with a minister in attendance. The manner of his dead was ungodly enough. The least I can do is ensure a few prayers are said over his remains.”

“That’s a generous thought, Samuel. But this poor, silent witness may have a tale to tell, yet.” The Doctor looked around at the TARDIS. Samuel Pepys had seen it before on his last visit to this century. It was after he had given up his famous diary, so there was no written record to upset historians of the future, and he had accepted that it contained scientific equipment. The Doctor was, after all, a fellow of the Royal Astronomical Society and The Royal Scientific Institute and several other honourable establishments. He was known and respected by Samuel’s other scientifically minded friend, Sir Isaac Newton himself.

But The Doctor didn’t think it a good idea for Samuel to see the interior of the TARDIS.

Fortunately they were distracted by the arrival of Leela, clothed once more in her velvet and satin with some of the late Mrs Pepys foundation garments beneath.

“Doctor,” she said. “The men you sent to the kitchen have been drinking too much wine. They’ve scared Mary-Anne away with ‘inappropriate actions’ and now they’re talking about taking a boat to search for the creature in the river. Do you think I should knock them all on the head and let them sleep until they see sense?”

“Tempting,” The Doctor replied. “But maybe not the best...”

“I’ll give them the coins I promised,” Samuel decided. “And remind them that this is the sixth body this week and that it’s started to snow again. That might put them off any such nonsense.”

Samuel slipped away to do his duty. The Doctor asked Leela to watch for his return then opened the TARDIS door. He whistled softly and K9 whirred quietly out into the cool basement.

“K9,” The Doctor said. “I need an analysis of an organic substance.” He took a scrap of the dead man’s clothing and pressed it close to K9’s sensor. It was covered in something yellow and noxious. The Doctor had a fair idea what it might be, but he wanted to be definite before he decided his next course of action.

“Substance is saliva of a Piscean lifeform,” K9 replied. “Specifically, Morphic Haggian Entity, commnly known as...”

“Yes, I know its common name,” The Doctor responded. “Good boy. You’d better slip back into the TARDIS, now. You’re an anachronism too many around here.”

“Master,” K9 replied and immediately returned to the TARDIS. The Doctor closed the door and covered the unfortunate body again before heading upstairs to meet Samuel and Leela coming from the kitchen.

“Samuel, I’ll need a couple of strong swords,” he said. “And the use of a rowing boat.”

“Mine is tied up by the Watergate,” Samuel replied. “It’s small, but sturdy. You’ll not find a boatman this night, though. They’ve all hastened away from the river. The rumours of an ungodly creature... panic has spread like the fire itself that came to this city in 1666. The last men who could be of use to you went off from my kitchen quite incapable of walking in a straight line, let alone managing a boat.”

“Not to worry,” The Doctor said. “I know a smart young man...” He looked around to see young Tom Haggerty dressed in a used but serviceable set of clothes including a stout pair of boots and the warmest overcoat he likely ever owned. “Tom... would a sovereign tempt you to one more trip on the river.”

The Doctor pulled the coin from his pocket. Leela was puzzled. She was full sure there were no coins like that among the contents he had spread out on the receptionist’s desk earlier. Tom was mesmerised by the opportunities open to him with a coin like that in his possession, but fearful of stepping through the York Watergate again.

“Two sovereigns,” The Doctor said pulling a second coin from his other pocket. “Master Pepys will hold them in his safe-keeping until we return.”

“You want me to remain behind?” Samuel Pepys was puzzled. “But... you asked for two swords... I thought...”

“Samuel, you’re nearly blind in the dark these days, and your kidneys trouble you in this cold weather. You’re safer here in your own warm house.”

Samuel conceded as much.

“But... do you mean... you are taking a lady... Mistress Leela... on such a desperate endeavour... surely...”

“I can fight as well as any man,” Leela protested.

“She most certainly can,” The Doctor assured his astonished friend. “The swords, if you would.”

Samuel fetched two swords. When he returned, The Doctor and Leela were wrapped in warm coats and mufflers and ready to face the unknown. Tom Haggerty had steeled himself for the adventure, and despite his words earlier, seemed almost raring to go into action. Samuel Pepys sighed dismally at having to be left out of the expedition, but The Doctor was adamant that he should not be involved.

“Poor old Sam,” The Doctor opined as they walked down to the river through the Watergate. “He’s only in his fifties, but he was plagued by renal trouble all his life and besides, he was never really a man of action. He’s seen half a century of turbulent English history from behind a desk. He would have done well on Gallifrey. Our civil service could use a conscientious administrator like him.”

Neither Leela nor Tom really understood what The Doctor was saying. But he carried on talking until they reached the low sided rowing boat tied up on the water’s edge. Snow had accumulated in it all day and there was a film of ice around it. The river would probably freeze up by morning. But for now it was flowing sluggishly. Leela sat in the stern of the boat and young Tom took up the oars. The Doctor sat at the prow. He told Tom not to light any torches but to use his own judgement as he rowed to midstream and then downriver in the direction of Blackfriars.

“Ah,” The Doctor said presently. “Tom, do you see that? The silvery glow trailing in the water. It’s a kind of phosphoresce...”

“Yes, sir,” Tom replied. “Like a cuttlefish bone.”

“Exactly like that. The creature we’re pursuing leaves a trail, but with torches blazing we’d never make it out. With every other boatman hiding in alehouses high above the water line, it’s dark enough for it to show up.”

“You... want me to follow it, sir?” Tom asked nervously.

“I do,” The Doctor answered. “But you’re not going to be in any danger, I promise you. Just row steadily until I tell you to stop.”

“Yes, sir,” Tom managed to reply. He did as he was told, rowing steadily with the tide for the length of some six hundred yards before The Doctor told him to pull into the shore.

“This is Somerset House,” The Doctor noted. “Interesting place. I should bring you on a more salubrious night, Leela. In the time of Queen Henrietta, it was known as Denmark House and there were some very exciting balls....”

“Sir...” Tom said. “The... silver... goes up the steps....”

“It goes down again, too,” The Doctor noted. “I think our quarry may have pursued a victim. Stay there, both of you and keep your eyes peeled.”

Leela wasn’t sure how to keep her eyes peeled. Tom understood the concept and peered around in the gloom for anything that might make him want to leap out of the boat and run for it, with or without his sovereigns.

The Doctor came back after a little while. He was carrying a body. He laid it in the water and let it float away downstream.

“Looks like one of the footmen,” he said. “Remnants of livery still on the skeleton. Poor fellow. His body should carry away on the ebb tide. When it turns up in a mudbank further down the river it’ll look like a simple drowning. Less distressing that way.”

“But the creature got away?” Leela asked.

“I’m afraid so,” The Doctor replied as he climbed back into the boat. “There, look. It slipped back into the water by the corner of the steps.”

Tom manoeuvred the boat back into the river again and rowed steadily with the ebb stream. He wasn’t particularly tall for his age, but he had a wiry strength and even when The Doctor offered to take over the rowing for a while he kept on going for another quarter of a mile along the River Thames to the place called Blackfriars where, in the era The Doctor and Leela visited first this day, there was both a wide road and a rail bridge. In this time there was neither.

But there was a wharf alongside the place where a narrower river joined the Thames.

“It goes that way, sir,” Tom observed. “Up the Fleet.”

“Known as the New Canal at this time,” The Doctor noted. “Come on, lad, you come and sit up front and rest and I’ll row for a bit. You must be exhausted.”

The boy agreed to the rest. The Fleet was quite fast flowing at the convergence, and even The Doctor had to row strongly against it. He recalled that this was not the first time he and Leela had entered the Fleet from the Thames. The last time, though, was in the late Victorian era when the much reduced stream, damned off at Highgate to form a series of reservoirs, had been covered over and trained through a noisome culvert with the city built over it. In this time it was a free flowing river. It was a little polluted, since it was used as an open sewer system by the populations of Kentish Town and Holborn, but it was only a little worse for that than the Thames itself.

It was just short of a mile upstream when the Fleet flowed down through Hampstead Heath, and it was there, Tom noted, that the silvery trail made landfall. The creature had emerged from the water on the Heath.

It’s path could be seen even more clearly on land, though. At first it was like following a slug trail. After a while, it looked more like footsteps. Leela and Tom both commented on the fact.

“It’s a Merman,” The Doctor explained. “Not what you might think of by that term, Tom,” he added. “There’s a romantic idea among humans about half-fish people. Artists portray them as beautiful maidens and strong, bearded men with tridents. But this is a very different creature. It has no business being here in this city. It should live in a place with abundant fish stocks, where it could cause no danger to sentient lifeforms. I just hope it’s prepared to be reasonable.”

“Reasonable?” Leela queried his choice of words. “Doctor... do you mean you’re not going to slay this monster?”

“I slay nothing if I can help it,” The Doctor replied. “You should know me better than that, Leela. If it’s morphed into a near Human form, then it will be able to communicate. If I can make it see reason...”

“Sir!” Tom yelped fearfully and pointed. The Doctor saw what he had seen and broke into a run. Leela hitched up her skirts and ran, too. The sword in the scabbard fastened at her waist jangled but she didn’t yet draw it. The Doctor hadn’t drawn his.

Tom looked at them both and then decided that with the monster on land, the boat, on the water, was a safer place to be.

The Doctor reached it first. The silver phosphorescent creature had the appearance of a very slender man with gangly limbs and a thin face that while having Human features managed to look fish-like. It walked uneasily, as if unsure how legs worked. Running away was not an option even before The Doctor called upon the creature to stop.

It stopped and faced them, baring long, sharp, white teeth. It raised its arms and grabbed at The Doctor’s neck with long fingered hands that felt cold against his skin.

“Let him go,” Leela hissed. The sound of her sword being drawn from the scabbard matched her tone. She stepped close enough to prick the creature’s neck with the point. “Let him go or I’ll cut your head off.”

The creature let go and stepped back, but still snarling and hitting out with its long reach.

“Calm down,” The Doctor ordered it. “I know you understand me. You’ve killed six men and assimilated their DNA. That’s why you’re able to walk on dry land. Oh, yes. I know what you are. You’re a Morphic Haggian Entity, commonly known as a Haggian Merman. You don’t belong on this world. I know a few of you have migrated here, but mostly you confine yourselves to remote parts of the ocean. You eat fish. You live like the fish, in the deep water. What are you doing here in a city? Why are you killing humans?”

The Merman replied in a rasping voice of something that was only getting used to vocal cords, describing how it had followed the hull of a boat that crossed the great ocean where it lived. The journey had been long, but it felt a compulsion to follow until it came upriver to London.

“Men are exploring much more of this planet than they used to,” The Doctor noted. “You must have come across one of the sailing ships returning from the ‘New World’. But that doesn’t answer my question. Why did you kill those men?”

“I kill to eat,” it replied. “The creatures of the water are my prey, my food.”

“Doctor, it is a hunter... like me.”

“Not at all like you, Leela,” The Doctor replied. “You never killed men for food.”

But that was because her people didn’t practice cannibalism. Leela’s people killed wild animals in the forests. Those animals might easily kill them for food if they were unwary. They were part of the food chain.

This creature killed for food. It was part of the food chain. On the river Thames, it was the top of that chain. Humans were below it just as cattle and sheep were to Humans.

“You can’t keep doing that,” The Doctor said. “The Humans will come in force and kill you. I’ll take you to a big ocean where there will be no people, but plenty of fish. You can live out your life that way. You’ll revert to fish life, without this intelligence, without this ability to walk on land and breathe air. But you should not have had that in the first place.”

“No!” the Merman rasped. “I like this form. I will feed on men and grow stronger.”

The Doctor and Leela both pressed forward with their swords, but the creature already was strong, despite its gangly appearance. It pushed them both away and began to run back towards the river. It seemed to have mastered the combination of muscles required for running, now, and it was fast.

Leela was nearly as fast, but she was still yards behind when the Merman reached the water’s edge. Then something dark rose up from the river in front of it. It fell back, stunned. The Doctor caught up and examined the Merman, pronouncing it out cold from a blow to the head with a blunt instrument. He looked up at Tom Haggarty, standing in the boat, still wielding one of the oars.

“Crude but effective,” he said. “Pass me that coil of rope down there beside you. A stasis field would be nice right now, but I’ll settle for some strong rope and my boy scout skill with knots.”

“You’re not goin’ to kill it?” Tom Haggarty asked. “While it’s down?”

“Tom, my boy, I hope you’re not the sort of coward who would finish anything off while it’s helpless,” The Doctor replied. “I’m going to take the creature where it can do no harm to humans. I told it I could, and I will. It would have been better if it had gone willingly, but it’s all the same in the end.” He put the Merman in the bottom of the boat. “Leela, you watch it in case it gets stroppy when it wakes. Tom, you row us down stream. I’ll take over when we get back to the Thames and we’re against the tide. Back to Master Pepys’ house for a spot of supper before we go our separate ways.”

When they got back to 12, Buckingham Street, the body of the unfortunate boatman had been removed to the city morgue. The Doctor put the still unconscious Merman into the TARDIS where he fixed him into the rarely used magnetic chair that held the sitter in place no matter how they struggled. He smiled wryly as he recalled the last time it had been used, to keep a young man called Steven Taylor under control until he realised that he was not the enemy. Two lives back and hundreds of Time Lord years. The Merman could stay there until he and Leela were ready to leave Samuel’s company.

Meanwhile, they were invited to supper along with young Tom, who was astonished to be invited to dine with his betters. The boy was still adamant that he was going to seek a new career far from any tidal rivers, and The Doctor gave him remuneration enough in the coinage of the times to get him started.

Much later, The Doctor and Leela said farewell to Mr Samuel Pepys and his household. Shortly after that, the TARDIS materialised in hover mode ten feet above a blue ocean.

“This is planet Earth before the continents split apart and the oceans separated,” The Doctor said as he released the Merman from the chair and brought him to the open TARDIS door. It is many millennia before sentient life will exist here. Until then, there is plenty of marine life in the water. You can live out your natural lifespan here.”

The Merman said nothing. He had accepted his fate. He stepped forward, teetering on the threshold for a moment before he dived into the deep, deep, blue water that teemed with life. The Merman would become part of the food chain, a predator among all the other predators of the see and its prey.

“That is the best I can do for him,” The Doctor said. “He will evolve to look more like the food he eats, and less like a Human. Even if his fossilised bones are one day found high and dry in the Sierra Nevada mountains he won’t be any more of a puzzle than any other extinct creature.” He turned and smiled at Leela who was puzzled by the talk of fossils. “How about we go and visit another old friend of mine, Mr Charles Darwin.”