"Nightmare in Silver" is Neil Gaiman's vision of a consolidated, constantly evolving Cyberman army, one that can upgrade at speed and overcome any obstacle, like a metallic virus.

It also shows the Doctor at war with a possessed version of himself, a challenge that fully tested Matt Smith's ability to play games with his face while getting important character details across.

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

(The episode is available on iTunes and Amazon.)

Neil Gaiman based his story on the idea that the two dominant strains of Cybermen — those that evolved from the planets Mondas ("The Tenth Planet") and Telos ("The Tomb of the Cybermen") and those created in a parallel universe by Cyrus industries — had somehow met and forged an alliance: "My theory is the Cybus Cybermen were sent to Victorian days and zapped off into time and space at the end of "The Next Doctor." They met a bunch of the Mondasian/Telosian Cybermen, and there was some cross-breeding and interchange of technology, which is why you then get the ones that look like, but actually aren't, the Cybus Cybermen. And then I thought well, they’re going to keep upgrading themselves – my computer doesn’t look like it did five or ten years ago, definitely not 15 years ago. It’s going to be faster and it’s going to be better. So let's make the Cybermen faster and slicker and better."

The end of the story was planned to be a version of the Cyberiad which showed all the various iterations of the Cybermen throughout Doctor Who history plotting to entrap the Doctor.

Naming the story proved to be problematic. It had the working titles "The Last of The Cybermen," "Silver Ghosts" and "The Saviour of the Cybermen," before Gaiman settled first on "The Last Cyberman" and finally "Nightmare in Silver," a condensed version of an earlier idea — "A Nightmare In Silver."

The idea of a Cyber-Planner came from 1968's "The Wheel in Space," which featured an unnamed and immobile central hub intelligence to coordinate Cyber-forces. Gaiman wanted to update this role, and give it some personality, opting to place it inside the Doctor's head rather than within a computer.

Gaiman's favorite Doctor at the time of writing the episode was Smith, despite having grown up loving Patrick Troughton's Second Doctor: "Matt Smith is amazing, and it's writing for him that gives me an increased awareness of how amazing. He is technically one of England's finest actors. Matt for me is the only Doctor who really feels a thousand years old and a kid – he really is this ancient space alien."

https://www.youtube.com/watch?v=r_PP8zSTEIw

Several moments of the plot were changed during the writing process, which also had to bear the loss of Gaiman's laptop. At first this was an adventure written for the Doctor's new companion — the Victorian governess Beryl that eventually became an early incarnation of Clara Oswald — featuring an assortment of alien circus freaks. These were later changed to an all-female punishment platoon, then a mixed brigade. The final showdown took place on an island, which was also called Natty Longshoe.

Speaking of which, the name Natty Longshoe was a nod to Astrid Lindgren's 1945 children's novel Pippi Långstrump, better known to Anglophenia readers as Pippi Longstocking. The name Nehemiam Webley came from the musician Jason Webley, a friend and musical foil of Neil's wife Amanda Palmer.

Gaiman wanted a Cyber-army that could do uncanny things, rather than just making a slow and inexorable advance: "There was a scene where I had a hundred Cybermen coming up out of the sea on somewhere like Brighton Beach, which is all pebbles, and then moving completely silently over the pebbles. One human is running away from them and you can hear the crunchy-crunchy-crunchy-crunchy of the pebbles, but they’re completely silent, which I really liked, the idea that you can't hear them coming at all."

Mr. Clever says, of the Doctor's psyche, that he has "had some cowboys in here." It's a phrase used by the Tenth Doctor to describe the dilapidated ship SS Madame de Pompadour in "The Girl in the Fireplace" and by the Eleventh Doctor to young Amelia Pond when presented with the crack in her wall in "The Eleventh Hour," and it'll be familiar to anyone asking a tradesperson to come and give a quote for a repair. The inference is that the problem is not a fault or breakage, but previous attempts to fix a problem, which have been ineptly handled.

There are some familiar waxworks in Nehemiam Webley's collection including a Gastropod ("The Twin Dilemma"), an Ultramancer, a Lugal-Irra-Kush and a Pan-Babylonian ("The Rings of Akhaten"), a Blowfish (from the Torchwood story "Kiss Kiss, Bang Bang") an Uvodni and a Shansheeth (from the Sarah Jane Adventures stories "Warriors of Kudlak" and "Death of the Doctor," respectively) and one of the ventriloquist's dummies seen in "The God Complex."

The idea of the chess-playing Cyberman as a carnival attraction was based on the 18th century attraction the Turk. It was made by Wolfgang von Kempelen in 1770, and gave the illusion that an automaton was a chess grand master:

https://www.youtube.com/watch?v=RdT4yG8wczQ

NEXT: 10 Things You May Not Know About 'The Name of the Doctor'

Now go back and read the entire 10 Things You May Not Know About Doctor Who archive.