"Oxygen" takes one of Doctor Who's best-loved story formats — the space station siege — and introduces very real jeopardy for everyone concerned. We see the Doctor struggling to protect his companion, and in the end making a very physical sacrifice in order to keep her safe, while traumatizing the rest of the crew he has pledged to help. It's a dark and claustrophobic tale, and a potent political allegory too.

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

While the episode starts with a very familiar line from Star Trek — "Space: the final frontier" — it's not the first time this combination of words has appeared in Doctor Who, in fact there's an entire Third Doctor adventure called "Frontier in Space" from 1973, which was shown seven years after Captain Kirk first uttered those words.

Ganymede Systems, the company that supplied the smartsuits, are named after Jupiter's largest moon, but it's not the first time that name has appeared in Doctor Who. It was the name of a space station in "Revenge of the Cybermen," and a tool — the ganymede driver — in "The Hand of Fear." And in the audio adventure "Immortal Beloved," the Doctor meets the mythical Ganymede, cup-bearer to Zeus, after whom the moon was originally named.

Temporary blindness has cropped up in Doctor Who before, although most commonly to the companions of the Fourth Doctor. Sarah Jane Smith was blinded by a flash in "The Brain of Morbius," and Leela, her successor, was blinded by the flash of an exploding Rutan ship in "The Horror of Fang Rock." As we've mentioned before, this was the explosion that turned her brown eyes blue, a decision taken because actress Louise Jameson had been wearing brown contact lenses but found they were hurting her eyes. More recently, Grace Holloway was temporarily blinded by looking into the Eye of Harmony in the Eighth Doctor TV movie, and Amy Pond had to keep her eyes shut when she was possessed by a Weeping Angel in "Time of Angels," although she wasn't actually blinded.

https://www.youtube.com/watch?v=TN7ZcyXWYHY

The trick the Doctor plays on Nardole with the fluid link harkens back to the second Doctor Who story ever, in which the TARDIS travels to Skaro. The First Doctor removes the link and claims that he needs to find mercury in order to make the TARDIS work and return everyone home. Assuming he hasn't since fixed the problem, the fact that Nardole can remove the fluid link without causing the TARDIS to stop working suggests that we've just discovered that he was lying about needing the mercury, just in order to explore the planet, and thereby meet his greatest enemies: the Daleks.

Bill isn't the first of the Doctor's companions to risk death in space, the First Doctor took Trojan servant girl Katarina aboard the TARDIS (in "The Myth Makers"). She was trapped in an airlock by the unpleasant Kirksen in "The Daleks' Master Plan," while he tried to blackmail the Doctor. Katarina chose to open the airlock instead, expelling Kirksen and herself into space.

Of the five guest actors in this story, three have appeared in Doctor Who or Bernice Summerfield audio adventures before: Kieran Bew (Ivan) was in "The Lady of Mercia" and "The Helm of Awe," while Mimi Ndiweni (Abby) was in Big Dig. Peter Caulfield, who played the blue-faced Dahh-Ren, played several parts in "Cold Fusion." Because he shares a first name and surname initial with Peter Capaldi (known as "Peter C" on set), Peter Caulfield was referred to by the crew as "Blue Peter," a pun on the long-running BBC children's TV show of the same name:

The Doctor's yellow yoyo makes a welcome return. First seen in the hands of the Fourth Doctor in "The Ark In Space," it later reappeared in "The Girl Who Died" and "Kill the Moon" from seasons 9 and 8, respectively.

The Doctor quotes from the Bible. He says "Death, where is thy sting?" a line from the First Epistle to the Corinthians in the New Testament.

When the teaser trailer for season 10 appeared at the end of the 2016 Christmas special "The Return of Doctor Mysterio," the scenes taken from "Oxygen" had to be doctored slightly. The Doctor's eyes appear free of clouding, so as not to spoil the fact that he damages his eyes in this story.

https://www.youtube.com/watch?v=o3x4YF3EisE

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