The Medusa Sacrifice started with a desire to re-use the Medusan Cascade that was so brilliantly used in the Doctor Who Season Four finale. There isn’t a lot of information about it, really, except that it is a sort of malleable part of space where reality isn’t what it ought to be.

In Sound of Drums, the 2007 penultimate episode of Doctor Who, The Master says that The Doctor sealed the rift at the Medusa Cascade. In Fires of Pompeii, Eveline the prophetess says to him "Your real name is hidden. It burns in the stars, in the Cascade of Medusa herself". So there’s some mythology and some history there. But as far as setting a story there is concerned, there’s still something of a blank page.

The idea came to me that sealing the rift requires some sacrifice, and that, when The Doctor sealed it the last time, it did actually cause the death of a fellow Time Lord. The name of that Time Lord was carefully avoided until the last couple of pages. Cal Lupus of the House of Oakdaene has appeared in a half a dozen Theta Sigma stories and is likely to be in a few more of them. He becomes a good, close friend to Chrístõ, and something of his right hand man. The idea that he might have stronger feelings for him only really came to me in context of this story. It probably won’t be apparent in the Theta Sigma stories.

‘People made of smoke’ is a phrase first mentioned by the seventh Doctor in that famous soliloquy in the last moments of the last episode of the series.

“"There are worlds out there where the sky is burning, and the sea's asleep, and the rivers dream. People made of smoke, and cities made of song. Somewhere there's danger, somewhere there’s injustice, and somewhere else the tea's getting cold! Come on, Ace — we’ve got work to do!"

I always wanted to get the ‘people made of smoke’ into a story, but never quite worked it out. Funnily enough, I’d already started to describe the formless lost souls when I remembered that quote.

The bonding of the Artron energy body to give Spenser his life back – even though it meant Cal was doomed to remain in the Cascade as a non-corporeal being, stretches the possibilities of what Artron energy can do. But there aren’t many limits to that. After all, Artron energy is what makes Jack Harkness immortal and we’ve seen now just how immortal he is after his resurrection in Children of Earth. What happened to Spenser is pretty simple in comparison.

The relationship between Spenser and Davie is one that is proving very durable. It will have to end one of these days, but there are a couple more emotional storylines for them both first. I can at least promise you they’re not going to go down like Captain Jack and Ianto Jones.