"Cold Blood" is the finale of Chris Chibnall's two-part reintroduction of the Silurians to Doctor Who, a story that also left the series with a performance by Neve McIntosh (playing the sisters Alaya and Restac) that she soon found herself being re-introduced to the Doctor as Madame Vastra. It's also the first time we see Rory dying, bringing to an end (for now) the neat little family the Doctor gathered around himself. It is, therefore, the first time Amy has to see it too.

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

(The episode is available on iTunes and Amazon.)

There's a crossover between the worlds of Doctor Who and The Hitchhikers Guide to the Galaxy, in that Stephen Moore, who plays the Silurian elder Eldane, is best known as the voice of Marvin the Paranoid Android from the radio series and TV show.

And if you think Stephen was hired as much for his mellifluous tones in Eldane's narration at the beginning of the episode, it might help to know his lines weren't actually added until after the episode had been shot. They weren't in the original script and were put in by Steven Moffat during the edit.

As things get tense towards the end of this story, the Doctor uses the phrase "squeaky bum time" (and "super squeaky bum time"), a moment of descriptive emotive language that was coined by Sir Alex Ferguson, the manager of the soccer club Manchester United, to describe a narrow squeak.

One of the scenes that didn't make it to the final transmission included a section in which Amy and Mo discovered a vivisection laboratory full of modern animals, including a dog which had been lost by Tony Mack years beforehand.

As the Doctor recovers from his decontamination, he says "Not got any celery, have you? No. No, not really the climate," which is a reference to the Fifth Doctor's habit of wearing celery on his lapel, having claimed it was an "excellent restorative" in "The Caves of Androzani."

Richard Hope, who played the scientist Malohkeh, is something of an expert in the art of being Silurian. As well as reprising his role as Malohkeh in "The Wedding of River Song," he played Bleytal, another Silurian, in "Dinosaurs on a Spaceship." His voice can also be heard in the Big Finish stories "Changing of the Guard" (in which he plays William Heaton), "The Bounty of Ceres" (Moreland) and "Damaged Goods" (Harry Harvey).

There was a cut sequence from this episode that appeared in Doctor Who Confidential, which shows the Doctor and Nasreen being marched into the main hall to be executed. The Doctor, in an attempt to commiserate with a worried Nasreen, says "last time I was executed it really put a blight on the day."

Chibnall gave two of the Silurians names derived from the names of former Doctor Who scriptwriters — Malohkeh for Malcolm Hulke and Restac for script editor Terrance Dicks.

There's a neat moment of technological coincidence between the Silurians and the Dalek-built robotic inventor Edwin Bracewell, who appeared in "Victory of the Daleks." Both use a gravity bubble. The Silurians use theirs to travel from their city to the surface of Earth, while Bracewell used his to make Spitfires fly in space. And the Doctor will use one again, in "A Good Man Goes to War."

If Rory looks particularly young when re-watching now, there's a reason for that. This season was shot out of sequence, with this story being shot just after "The Eleventh Hour," Arthur Darvill's first appearance as Rory. As he filmed his death scene at the story's climax, Arthur knew only that Rory was scheduled to appear in "The Vampires of Venice" and "Amy's Choice," both scheduled to appear before this story, with no clue that Rory would be back.

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