"Midnight" is one of those special Doctor Who stories that stand apart from the main narrative of the mad man, his blue box and his best friend. It's a tense drama about human nature under pressure and it was written, aptly enough, by Russell T Davies, a human under pressure to come up with a replacement script to help flesh out his final full season at the helm of Doctor Who.

Here are a few things to keep an eye out for, the next time you watch:

(The episode is available on Amazon and iTunes.)

The script for "Midnight" was written when Russell T Davies realized that the story Tom MacRae had submitted — called "Century House" — was too similar in tone to "The Unicorn and the Wasp," in which the Doctor meets Agatha Christie. The story for "Century House" sees the Doctor appear on the reality TV show Most Haunted, investigating ghosts.

Russell told Doctor Who Monthly: "Tom's script would have been lovely, but it was too close, in too many respects, to the Agatha Christie episode... and I got a bee in my bonnet about this 'Midnight' idea. It was like an itch. It burnt in my head. It was a brilliant idea that just had to be written."

Russell has also said that his inspiration for the story came from an episode of Star Trek: The Next Generation which he has never seen. He explained to SFX magazine: "I've seen lots of Star Trek: The Next Generation, I think it's a lovely show – but there's one episode, the billing for which is so fascinating I've actively avoided ever seeing it. I love the idea so much, I'd rather think about it. Forever. The episode is called 'Darmok,' and the synopsis simply says that Captain Picard is trapped on a planet with an alien who can only talk in metaphors. Wow. That sounds brilliant. How does that work? What happens? How does it end? I've got no idea – not seen it! But it keeps resonating with me."

He continued: "In 2008, I wrote a Doctor Who episode called 'Midnight.' Is it like 'Darmok'? I don't know. But stripped down to its essentials, it's a story about a hero, an alien, and words. That's practically the same billing. Maybe the two shows are profoundly different, but I know for a fact that all those years of wondering about 'Darmok' led me to that script."

Another inspiration was the 2003 horror movie Jeepers Creepers II, in which a high school basketball team is trapped on a bus, terrorised by a monster. It was on TV while Russell was working on the story for "Midnight."

Keeping up with Season Four's comedy casting choices - every episode features an actor best known from a fondly regarded British comedy show — "Midnight" originally cast Sam Kelly as Professor Hobbes. Kelly will have been known to British viewers as the bespectacled German officer Heinz from the BBC wartime farce 'Allo 'Allo:

https://www.youtube.com/watch?v=8p9R9B0OPAE

Sam Kelly broke his leg shortly before production, so director Alice Troughton cast her namesake David Troughton (no relation) as his replacement with just two days' notice. David is the son of Second Doctor Patrick Troughton, and had appeared in Doctor Who three times before. Once as an extra in the Second Doctor story "The Enemy of the World," Private Moor in his regeneration epic "The War Games," and King Peladon in the Third Doctor story "The Curse of Peladon."

He is also a stalwart of Doctor Who audio dramas, and currently appears in the long-running BBC radio drama The Archers, playing Tony Archer, father of Tom Archer who is played by his son (and grandson of Patrick) William Troughton.

"Midnight" is the first televised Doctor Who story not to feature the TARDIS since the Fourth Doctor epic "Genesis of the Daleks" from 1975. The only other stories that don't feature the Doctor's ship in some form or other are all from the era of the show in which the Third Doctor is exiled on Earth after his second regeneration: "Doctor Who and the Silurians," "The Mind of Evil," "The Daemons," "The Sea Devils" and "The Sontaran Experiment."

That said, "Mission to the Unknown" is a 1965 standalone episode that acts as a prelude to the First Doctor story "The Daleks' Master Plan." All of the action takes place away from both the Doctor and the TARDIS, and neither makes an appearance.

Another reason for changing the scripts was that this was intended to be the fiftieth episode of the relaunched Doctor Who, and that required something special. Working titles included Crusader Five, and then, to mark the special occasion, Crusader 50, which became the name of the shuttle bus. However, during the production run the episode was moved to the spot between the two two-part stories at the end of Season Four, which undercut the celebration somewhat. The 50th episode is actually "Silence in the Library."

Ayesha Antoine, who played Dee Dee Blasco, has an alternate life playing Ruth in the Big Finish Doctor Who spinoff series of Bernice Summerfield audio stories. Brilliantly, for "In Living Memory," Ruth is brainwashed by a race called the Epoch into believing she's an actress called Ayesha Antoine.

Russell T Davies is a great fan of pop music, but in the case of "Midnight" he picked his song more for irritation than delight. "Do It Do It Again" is a 1977 British chart hit by the Italian singer Raffaella Carrà. British viewers may have assumed this was a long-lost entry for the Eurovision Song Contest, but Raffaella has maintained a worldwide career as a singer, actress, dancer and TV presenter, across Europe and Latin America:

https://www.youtube.com/watch?v=i6TFEv5mgzs

And finally, while you may know that Colin Morgan, who plays Jethro, went on to far greater fame, taking the title role in the BBC's Merlin, you may not be aware that he had worked with Catherine Tate before he appeared in Doctor Who. In the 2007 Christmas special of her sketch show, Colin played John, a good Northern Irish boy whose hard-as-nails mom (Catherine) was firmly welcoming of his coming out as gay. That character's catchphrase, "Our John is a big gay man now" features prominently.

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