Mark Gatiss reveals the plot of the Doctor Who episode that never was

“Essentially it came about, because I was going to do a sequel to Sleep No More,” Gatiss said, explaining that he fancied following in the footsteps of classic Doctor Who stories The Abominable Snowmen and The Web of Fear to revisit his Sleep-themed monsters as those stories did with robotic Yeti.

“I had this idea [that] I'd like to do the double in a kind of Yeti way, to have two stories about the same monster,” he said.

"Although it was still in space, [Sleep No More] was originally going to be on a trading floor; it was a stocks and shares thing, with these executives who were trying to stay awake in order to be more productive.

“And then it sort of changed into the sleep research station [of the finished episode].”

Still, despite the changes to his pitch, Gatiss held on to his original idea for the episode, thinking that it could instead work for a sequel in series 10.

“I thought there was something in that, and actually maybe I could do a modern day one, set in the City,” he said. “Where they'd invented the same process but actually thousands of years earlier, and it had the same effect.”

In the end, though, the news that both star Peter Capaldi and showrunner Steven Moffat were leaving the series made Gatiss rethink what could be his very last Doctor Who script (he’s not sure if he’ll be asked to contribute under new showrunner Chris Chibnall), instead turning to some of his favourite themes for what became Empress of Mars.

“I thought, 'Well you know, everyone's going',” Gatiss recalled. “And so I just said to Steven, 'Can I do the story I've always wanted to do, which is the Ice Warriors on Mars?' And he said 'Yeah!' So that's how all this came about.

“You know, I started with the Victorians. I've come full circle,” he added, referring to 2005 episode The Unquiet Dead. “It's kind of pressing all my buttons this one.

“It's the story I've always had in the back of my mind to do, I suppose, and it's just so nice to see loads of Ice Warriors. I mean, it's just never happened before. And a new one – and an Ice Queen. So yeah, I'm very chuffed with it.”

Doctor Who continues on BBC1 this Saturday 10th May at 7.15pm