"The Doctor Falls" is a treasure trove for references that span the 50-odd years since Doctor Who first hit TV screen. The Master faces a future version of himself who tells him she loved being him, just as the Tenth Doctor says to the Fifth in "Timecrash," multiple iterations of Cybermen fight side by side, and Nardole finds himself reacting to a romantic advance by saying someone is "only human," just as the Tenth and Eleventh Doctors used to.

And it is also a hugely moving ending to Bill Potts' travels with the Doctor. Here are a few things to keep an eye out for, the next time you watch:

“Back in time for tea” puts Bill firmly in the camp of British people who call their meals breakfast, dinner and tea. This is stereotypically more common in the north of England, with the southern equivalent being breakfast, lunch and dinner.

The Doctor refers to himself as "the Doctor, the original you might say" to the Cybermen, something the First Doctor also says right at the end. This line first appeared in the 1984 multi-Doctor story "The Five Doctors," said by the First Doctor. As a further note of continuation between the two stories, "The Five Doctors" was the only other televised Doctor Who story in which the First Doctor was played by someone other than William Hartnell.

The Doctor's reassuring offer of a jelly baby is a throwback to one of his favorite sweets. The Second Doctor was the first to be seen carrying a tatty paper bag of jelly babies on his person, particularly in the multi-Doctor adventure "The Three Doctors," but it's the Fourth Doctor who was most generous with them, often using them to confuse angry guards on first meeting people in a new environment:

https://www.youtube.com/watch?v=P6MKbbFgMUY

The Master calls the Doctor "Doc" to annoy him. Presumably he knows that the First and Sixth Doctors both took haughty pains to correct anyone who spoke to him with such familiarity.

The Doctor's first words when waking up after being brought back to life are "Sontarans! Perverting the course of human history!," which were not only the Fourth Doctor's first words after his regeneration (and a comment the Twelfth Doctor made in similar circumstances in "Listen"), it's also the plot of the Third Doctor adventure "The Time Warrior."

When Missy and the Master are dancing on the rooftop, their soundtrack is "Midnight, the Stars and You" by Al Bowlly. The same song also appears in Stanley Kubrick's celebrated horror film The Shining.

https://www.youtube.com/watch?v=vQfJKmXm_7c

As the Cybermen attack, the Doctor lists some of the previous locations in which he has bested their kind, including Telos ("The Tomb of the Cybermen"), Voga ("Revenge of the Cybermen"), Planet 14 ("The Invasion"), Canary Wharf ("Army of Ghosts" / "Doomsday") and the moon ("The Moonbase"). He also mentions Marinus, a source of an alternate origin story for the Cybermen, in the comic adventure "The World Shapers".

The Master quizzes the Doctor on the ways in which he has died, asking if he's ever burned to death (which the Master did in "Planet of Fire"), or drowned. We know the Doctor did drown in Donna Noble's alternate reality in "Turn Left." And Missy's comment that she knows he's died by falling refers to the Fourth Doctor's regeneration in "Logopolis," giving an extra layer of meaning to the title of this story:

https://www.youtube.com/watch?v=l3w-BHsXt4I

Speaking of which, the sequence where the Doctor's recent companions call his name, starting with Rose Tyler and ending with the Master (but leaving out Rory and Mickey, sadly), echoes a very similar series of visions experienced by the Fourth and Fifth Doctors as they were about to regenerate.

Steven Moffat chose floor 507 as the place where the final showdown would take place because Russell T Davies was fond of using 57 as a nice random-sounding number: "When people want to exaggerate a number, they always have a favorite number. Russell always uses 57, always 57.

I once eventually said to him, ‘Do you have a preference for Heinz products?’ – he didn't even know what I was asking him – 'Because it’s always 57! Even in scripts, if you’re exaggerating, you always say 57. That's your go-to. So it’s Floor 507 in tribute to Russell."

And one extra one. Did anyone else notice the Doctor — now a confirmed fan of Frozen — saying, "Doctor, Doctor, let it go,"? Surely a deliberate reference to that film's greatest hit?

https://www.youtube.com/watch?v=L0MK7qz13bU

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