So, now we know what lengths the Doctor is prepared to go to in order to keep the peace, and why no one, not even the daughter of one of his oldest friends, can entirely trust him to tell them the truth.

The Zygon Inversion may be a political allegory, a plea for personal tolerance or a scathing summation of the events that lead up to war, but it is also riddled with little hints and references that need a little explanation. Here are just 10:

As has become a common theme in this season, the Doctor renames an adversary with a nickname based on a British TV personality. This time he refers to Clara's Zygon doppelganger Bonnie as Zygella, a pun on the TV chef Nigella Lawson.

Seasoned Doctor Who fans will have thrilled to hear Kate say the words "five rounds rapid," as they come from a much-loved quote from her father, the Brigadier Lethbridge-Stewart. Watching the animated gargoyle Bok cavort in the Third Doctor adventure "The Daemons," the Brig orders him to be shot, with an implacable "Chap with the wings, there - five rounds rapid."

https://www.youtube.com/watch?v=0grVDUL_en0

It was such a popular moment for fans of classic Who that Nicholas Courtney (who played the Brigadier) even used it as the title for his autobiography.

Last week it looked as if the Z67 gas that can turn Zygons inside out was created by the Doctor's former companion Harry Sullivan. This has now been confirmed with a reference to his name, but you may wonder why the Doctor calls him "the imbecile." After Harry accidentally started a landslide and triggered a booby trap when attempting to remove a bomb in "Revenge of the Cybermen," an exasperated Fourth Doctor shouted "Harry Sullivan is an imbecile!" So even though Sarah Jane revealed that Harry went on to invent many successful vaccines after he left the TARDIS, it's clear that the Doctor is happy to stick with his first impression.

The Black Archive appears to have been updated since "The Day of the Doctor," as there's a very prominent Mire helmet on display while the Doctor, Bonnie and Kate bicker over the fate of the world.

When Osgood says there have been more than one meaning for the acronym TARDIS, she may be referring to a slight but repeated confusion as to the fourth word. It was stated right back in the first ever episode that TARDIS stands for "time and relative dimension in space," but sometimes that explanation has used the plural dimensions instead, which changes the meaning a bit, but not a huge amount. There's also Jackson Lake's "Tethered Aerial Release Developed In Style" (from "The Next Doctor") for completeness's sake.

"Totally And Radically Driving In Space" is entirely new, of course.

There are a couple of apt literary precedents for the phrase "brave new world," as sardonically used by the Doctor when asking Bonnie about the aftermath of her putative revolution. It's a quote from Shakespeare's The Tempest, in which young Miranda, who lives with her magician father Prospero on a remote island, first discovers other humans and, marveling, says, "Oh brave new world that has such creatures in it." Aldous Huxley borrowed the phrase when he created his own future dystopia Brave New World.

The Doctor is not the only British hero to employ a Union Jack parachute (whether for disguise or other reasons). The 1977 James Bond movie The Spy Who Loved Me also features a patriotic parachute, and when Bond encouraged the actual Queen Elizabeth II to jump out of a plane in Danny Boyle's opening ceremony to the 2012 Olympic Games, their chute also had a Union Jack design.

The Doctor's slight American accent and use of the phrase "and I mean that most sincerely"—when confronting Bonnie and Kate Stewart—are both affectionate nods towards the British TV presenter Hughie Green, who spoke with a similar showbiz burr and had that catchphrase. His most notable show was the talent contest Opportunity Knocks.

"Gotcha" is peppered across the script of this episode like, well, pepper. Previously heard as a private joke between the Eleventh Doctor and Amy Pond—she first said it reassuringly to mean "I've got you," in "The Beast Below"—the Doctor uses it twice to to mean that he's taken the upper hand. Once with Bonnie when he works out where Clara's body is being stored—"OK, we'll have to try something else. 20 questions. Where's your pod? Is it in a tunnel? Is it in London? Thanks very much. Gotcha!" and once with Bonnie and Kate, when he realizes Bonnie has started listening to what he has to say: "No one should have to think like that. And no one will. Not on our watch. Gotcha!"

But did you spot that seconds before Osgood reveals that she's now two people again with her own triumphant "Gotcha!," just after she jokes that Clara can let the Doctor die if he's really annoying, Clara says "got ya"? We'll all be doing it soon.

Two things seem to be alluding to the idea that the Doctor already knows that Clara's tenure in the TARDIS is about to end (or may even have already ended?). The Doctor refers to the time spent thinking that Clara was dead as the "longest month of my life," and when Clara queries this, saying it can only have been five minutes, he says "I'll be the judge of time," enigmatically.

Then there's the thing he says about Clara getting into your head: "She doesn't leave." Worth keeping a note of? Who can say...

13 Things You May Not Know About ‘Sleep No More’

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