The end of 'Doctor Who' Series Nine is almost here, but not before one of the most brilliant episode ever written in the history of the franchise. 'Heaven Sent' focused solely on Twelve as he dealt with Clara Oswald's death, and as showrunner Steven Moffat admits to Radio Times, it's a crazy (if not amazing) episode.

"It's a mad thing to do. I'm yet to find out whether I've just gone mad or not," Moffat said, adding that the idea for 'Heaven Sent' happened while he was working on a different episode.

"It occurred to me that I had just written a huge monologue for him at the end of [episode eight, 'The Zygon Inversion']," he said. "Ten minutes. I was typing forever in that. And I just thought, 'maybe we don't need anyone else - we don't need to cut to anyone nodding - it's just him?'"

Steven Moffat also expressed the need to keep things fresh in 'Doctor Who' due to the length of time that the series has been on air.

"It just puts it in a different place. If you're not going to try things that scare you a bit, that scare everybody a bit, then it'll feel what it must never feel - it'll feel like an old show," he said, adding that the former 'Doctor Who' executive producer had given him specific instructions.

"I remember when Russell T Davies first brought it back, he said, 'we can't ever make it feel old. It always must feel like a brand new show.' And that worked, and we carried on," Moffat shared, although he does admit it is a difficult task.

"We've now got to the point where the new show is also old. Slightly marred by the 50th anniversary being two years ago, but the new show is ten years old," he said. "So you're fighting that as well. You want to be doing things that are first/second series experimentation."

'Heaven Sent' is the first 'Doctor Who' episode to feature the Doctor without a Companion. The episode only features Peter Capaldi as the Twelfth Doctor and Jami Reid-Quarrell as the Veil, although Jenna Coleman's voice can be heard as Clara Oswald/The Doctor's Conscience.