Aztecs is one of my favourite First Doctor stories, and I really wanted to revisit the concept, but not revisit the Aztec times, as I really don’t know enough about them. I had the idea of Cameca’s mummified remains being part of a museum exhibit, though, and some kind of mystery around it. Initially it was going to be the British Museum in London, then I researched Mexico and find Chapultepec Park and the Mexican National Museum of Anthropology, and I knew I had my location. Several pieces of information came together after that. First the poet king Nezahualcoyotl fell into my lap, with his wonderful poetry that so perfectly describes The Doctor’s own philosophy of life – live it to the full and be magnificent. Then there was the ‘Umbrella’ in the foyer of the Museum. That didn’t need to be in the story at all, really, but when I saw something that reminded me at the same time of the Welsh National Assembly Building and the TARDIS console, the Welsh companions had to remark upon it.

The Aztec sun calendar, is, of course, one of the treasures in the museum.

Thalia Torres, was a name that came together for me. I looked up Mexican girls names and found Thalia, which is actually the name given to the Gallifreyan Lady High Councillor who condemned The Doctor in Arc of Infinity. That was fate. Then I discovered that Torres was also a Mexican surname, and since Fernando Torres is the top scorer at Liverpool FC this season that seemed right, somehow. She fulfils the sort of role the lady medical examiner in Men In Black has or the lady who looks after the whales in Star Trek, Voyage Home. She is the ordinary person who becomes mixed up in the madness of something extraordinary because she can’t help asking questions and seeking answers. She recognises The Doctor as an expert on her Aztec Lady. He recognises in her his own curiosity and wonder and thirst for knowledge – the thrill of the chase.

The technology I describe to reproduce Cameca’s face on computer, is, of course, fully available to forensic science and to some museums. Whether the Mexican one has such equipment, I don’t know. I hope it does.

The monument to the "Niños Heroes" that is mentioned in the story isn’t strictly part of the Aztec culture I wanted to champion. It belongs to the later, Spanish Mexican history, after the Aztecs had been wiped out by the Europeans. But their story is so tragic that I wanted to get it in there in the hope somebody might want to look further at it.

This was a lovely story to write because the history I researched was so rich and rewarding. It made it a joy to do.