In "Extremis" we find out a few unknown biological facts about the Doctor and Time Lords in general. We discover they have (or can have) multiple brain stems, that they can borrow physical qualities from future regenerations, with potentially catastrophic results, and that it takes one to kill one.

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

When discussing the Gallifreyan device to restore his sight, the Doctor uses the British phrase "swings and roundabouts" to explain his cavalier attitude to the potential sacrifices involved. It's a shortening of a longer phrase, which goes something like "What we lose on the swings, we make up on the roundabouts." It's thought to be derived from an expression used by carnival runners describing how the losses from one of their attractions would be offset by the gains on the other, roundabout being the British equivalent of a merry-go-round.

At CERN, Bill's first view of the scientists' countdown clock reads 5:15. This is, coincidentally, the billed time of broadcast for the very first episode of Doctor Who, in 1963.

Once the Doctor and Angelo have finished saying "Truth in the heart of heresy" / "And death in the heart of truth," Nardole makes the doleful observation that they would be "wizard" at writing Christmas crackers. This is a reference to the little slips of paper, with a joke or a light-hearted philosophical quip, which regularly appear inside crackers (note: they're not biscuits) on British dinner tables during Christmas day.

The Doctor pledges to guard Missy's body "on my oath as a Time Lord of the Prydonian Chapter." This is a reference to a particular sect of Gallifreyans, to which both the Doctor and the Master/Missy belonged. Their dress robes are a deep red, which suggests the Prydonian Chapter is somewhere between a fraternity, an old-school-tie network and one of the houses in Hogwarts. In the Fourth Doctor adventure "The Deadly Assassin," a Time Lord named Spandrell claims "when a Prydonian forswears his birthright, there is nothing else he fears to lose."

The Monk was voiced by Tim Bentinck, a regular cast member of Doctor Who by Big Finish audio adventures, and the 12th Earl of Portland. He's the second British peer to turn up in Doctor Who, after Floella Benjamin (Baroness Benjamin played Professor Rivers in several episodes of The Sarah Jane Adventures). But for British radio fans, he's most famous for his ongoing role as David Archer in the BBC Radio 4 continuing drama The Archers.

Joseph Long, who played The Pope, already has an iconic place in Doctor Who history. He played Rocco Colasanto, the Italian newsagent who was relocated to a shared house in Leeds with Donna Noble and her family, when London was destroyed in "Turn Left." He is later taken away to the Emergency Government's labor camps, and bids a distraught goodbye to Donna Noble and Wilf.

As our roundup of the most ferocious Popes of history points out, Pope Benedict IX was many things, and the subject of many a wild rumor, but he was not a woman. There was a very popular 13th century story about a woman (known as Pope Joan) who had disguised herself as a man and become the Pontiff, but it wasn't Benedict.

https://www.youtube.com/watch?v=SjYxwo0GcpM

Bill's spontaneous gasp of "Harry Potter!" as she and the Doctor enter the Haereticum (not a real place, unless it is and it's secret) is not without foundation. Members of several religious orders, including Protestant, Catholic, Muslim and Orthodox Christians have argued against the series and the depiction of magic, with the Vatican newspaper L'Osservatore Romano giving a full page to the debate, which ultimately resulted in the American version of Harry Potter and the Philosopher's Stone being renamed Harry Potter and the Sorcerer's Stone. Bill's comment—and the Doctor's rebuke—is a playful swipe at the controversy.

This is not the first time the Doctor has complained about Herman Melville's Moby-Dick. In the Doctor Who short story "Sunday Afternoon, AD 848,988" (from the anthology Short Trips: Destination Prague by Paul Crilley) the Seventh Doctor tells Ace he has tried over a hundred times to read the book, but never got past the first ten pages.

Nicolas, the CERN scientist who speaks to Bill and Nardole, quotes from the brilliant satirical singer and songwriter Tom Lehrer when he says, "We will all go together when we go." This is the title of this particularly cheery song about nuclear apocalypse:

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

NEXT: 10 Things You May Not Know About ‘The Pyramid at the End of the World’

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