Calla-Callix-Cassifrun was a title generated randomly by a little website called ‘planet namer’. I use it only rarely because the names it comes up with are a bit silly a lot of the time. But I was at a loss for a title for this place and that one had a bit of a ring to it.

This germ of this story grew from the idea I had of Donna in a big fur coat being sucked out of a window that had blown out on a space ship or space station, and the coat holding her long enough for The Doctor to rescue her. It didn’t originate from anything in particular, just a nicely complete vision that formed in my head. Then I had to think up a story in which it might happen.

There is a sort of rule of thumb in fiction, that it isn’t plagiarism but homage as long as there’s a reference in there. So after a series of sinister accidents involving machinery and technology on the space station have started to look like more than coincidence, somebody has to mention Stephen King, who wrote a whole series of short stories about possessed machinery – most of them rather dully similar, in fact. Of course, it would be Donna who brings it up at just the right moment, in the middle of a slightly rambling anecdote.

The ghastly accidents involving haywire transmits and water turning acidic were the products of my own imagination. So were the Exoan entities, although I quickly realised we needed another homage at this point, because they do seem to rather resemble gremlins. Only a little one, though. Because gremlins were about long before the film of the same name, so they can’t complain.