In an October 2007 email, Davies gave his first summary of his basic idea for the episode:
“The Doctor goes on a sightseeing trip, in a tank-like bus. But you don’t see the outside. No models, no CGI, other than a planet founder. It, literally, takes place on a set – inside the bus – which will have to seal its windows, so let’s say there’s a big old powerful x-ray sun outside. So it’s a box. It’s a show in a box. It will help our budgets right now.”
With that budget-saving premise and the general idea of a creature possessing people through their speech, Davies went to work on the script, calling it “Crusader 50” most of the time, a wink. take a look at how this would be the fiftieth episode of revival. He called himself “like an idiot” for taking on the project, because thanks to the deletion of the original Tom MacRae episode, he only had three days to complete it. “Preparation for 4.8 starts on Monday, so that’s a convenient deadline,” he wrote. “If that doesn’t work…well, I can keep it as a favorite project; something will happen to him one day. »
He ended up sending the first forty pages to producers Phil Collinson and Julia Gardner on Sunday night, giving them the option to go ahead with that script or stick with the original “Century House.” He told them: “This, or Tom MacRae’s! Don’t make up your mind until you read the attached document. It’s a strange scenario… But I swear sounds right, if you just let it flow. It’s all about atmosphere. It depends on the tension.” Sure — though it definitely didn’t seem like a sure thing at the time — Collinson and Gardner loved it.