Several ideas rolled into one. First, I wanted a story in which the twins and The Doctor explored a planet without Rose, so I gave her flu and had The Doctor explain that Time Lords don’t get flu. Ironically, the week I wrote this, Christopher Eccleston missed collecting his Most Popular Actor award at the National Television Awards – For playing Doctor #9 because he had flu symptoms. Sorry, Chris.

The next idea was to explore the idea of a race who had lost their lives in the Time War, playing on The Doctor’s conscience because of his own involvement in that universal cataclysm. It has never been categorically stated that he was the absolute sole survivor of all the races involved, only his own, but he seems never to have met any other veteran of the Time War, and it DOES weigh heavily on him. Rightly he asks the question if he should atone for his survival forever.

Having introduced these bodiless ‘sleepless souls’ the idea naturally followed to re-introduce the café people of Beta Delta IV and address the plight of their former enemies, the ones who had lost their souls. Putting the two together becomes an inevitable idea.

Symbiotic races, of course, are not new in science fiction. Two obvious examples are the Dax of Star Trek, Deep Space 9 and the Goa'uld and Tok’ra of Stargate SG1. These fall into two particular types. Dax and the Tok’ra are true Symbiots, sharing the life and experience equally. The Goa’uld, are parasites who take over the body of their host. (In the 2006 episode, Tooth and Claw the werewolf was a similar parasite using a Human body.)

The Doctor certainly intended for the meeting of the soulless and the sleepless souls to be of the first kind. His idea is to give the soul stealers a second chance and to help the innocent victims at the same time.

The last sequence, between Rose and The Doctor, is simply to address certain issues about their relationship. She still sleeps in the cabin bed as if her life in the TARDIS is a temporary arrangement, and The Doctor does his best to assure her that he wants her to be with him forever.