1. The Tenth Planet

Not exactly a mysterious easter egg, this, but the Christmas special sits closely alongside Doctor Who’s First ever regeneration story Twice Upon a Time, opening the episode with footage from First Doctor William Hartnell’s final adventure and taking place between two scenes towards the end of that serial (specifically between scenes 21 and 22 of part four).

Alongside Bradley, new actors Jared Garfield and Lily Travers play classic companions Ben and Polly in short cameo scenes (the roles originally played by Michael Craze and Anneke Wills) while the Snowcap base and Tardis sets from the episode was recreated in full by the Who production team (including some of the original props which were dug out of storage).

Bradley’s First Doctor morphs back into Hartnell’s for scenes appearing towards the end of the new episode, with the footage, original cast and episode writer rather sweetly credited alongside the usual cast credits. We've gone into some more detail about the new episode's Tenth Planet connections here.

2. The return of Villengard

As we revealed a few weeks ago, a throwaway gag from Steven Moffat’s very first Doctor Who story became an important location in his last one, with the weapon forges of Villengard – referred to as the place Captain Jack Harkness gets his sonic blaster before the Doctor blew the factories up – making their first onscreen appearance as the new home of one of the Doctor’s old frenemies (of which, more later).

You can read the original exchange from Moffat’s 2005 episode The Doctor Dances below, and read more about this callback here.

DOCTOR: Sonic blaster, fifty first century. Weapon Factories of Villengard?
JACK: You’ve been to the factories?
DOCTOR: Once.
JACK: Well, they gone now, destroyed. The main reactor went critical. Vaporized the lot.
DOCTOR: Like I said. Once. There’s a banana grove there, now. I like bananas. Bananas are good.

3. Soldier soldier

Longtime series writer Mark Gatiss, who has also acted in different parts in modern Doctor Who, gets one final role here as the WW1 Captain the two Doctors attempt to save – but his opposite number in the German forces, with whom he has a tense stand-off during the special, is also an experience NuWho writer, played by Being Human creator Toby Whithouse (whose Who credits include School Reunion, The Vampires of Venice and The Lie of the Land among others).

“I asked Mark a long time ago to make sure he’d be available and then I needed another actor to lie in a bomb crater and talk in German,” Moffat told Radio Times some weeks ago of the casting.

“I thought, ‘Wouldn’t it be great if we got Toby?’ who is also a very fine actor. So we have the two principals of my writing room sitting with guns drawn on each other.”

4. Hidden names

Blink and you’ll miss it, but one of the labels on the First Doctor’s Tardis console actually contains the name of visual effects designer Bernard Wilkie, who worked on six Doctor Who serials in the eras of Second and Third Doctors Patrick Troughton and Jon Pertwee and was known as a pioneer in his field.

And we wouldn’t be surprised if there was some more notable names hidden among those Tardis controls – even if it’s hard to spot them in the finished cut.

5. The First Doctor’s nicknames

At various points throughout the episode Capaldi’s Doctor pokes fun at Bradley’s incarnation with nicknames, at one point calling him Mary Berry (presumably due to his shared white-haired style with the former Bake Off judge) and at another suggesting they go “up and at them, Corporal Jones” in reference to the ageing Dad’s Army character played by Clive Dunn.

Interestingly, Dunn actually has a connection with Doctor Who having played a parody of the First Doctor in comedy sketch show It’s a Square World only 39 days after Doctor Who’s first episode had aired on TV.

And another of the Doctor’s nicknames for Bradley – “Mr Pastry” – is also a deep cut Who reference, calling back to the bumbling children’s character played by Richard Ahearne, an actor who was actually offered the role of the Fourth Doctor following the departure of Jon Pertwee (in the end, the part went to Tom Baker instead).

6. Browser history

A running gag about the Doctor’s internet usage reappears in this episode when the First Doctor wonders what illicit content is being offered to him through the Doctor’s sonic sunglasses. The browser history joke has previously cropped up in 2015’s The Zygon Inversion and 2017’s Smile.

7. New Earth

Professor Helen Clay, aka "the glass lady" from Testimony is revealed to have been from New Earth, the planet that appeared in 2006 and 2007 Doctor Who episodes New Earth and Gridlock, during the tenure of David Tennant’s Doctor.

It’s noted on a screen in the episode that Clay is from the year 5,000,000,012, which is a little earlier than both those episodes (which took place in 5,000,000,023 and 5,000,000,053 respectively) but later than 2005 episode The End of the World, which took place in the year 5 billion more or less exactly and kicked off the loose New Earth trilogy in ex-showrunner Russell T Davies’ era.

8. The Brigadier

At the close of the episode it’s revealed that Mark Gatiss’ Captain has a special link to a figure from classic Doctor Who –fan-favourite character Brigadier Lethbridge-Stewart (played by Nicholas Courteney), who acted alongside various Doctors over the years (making his last appearance in a 2008 episode of spin-off the Sarah-Jane Adventures) and who is, as it turns out, the Captain’s eventual grandson.

As we learn later in the episode, the Captain’s full name is Captain Archibald Hamish Lethbridge-Stewart, and Gatiss said at a recent screening for the episode that he was delighted to bring the character to life.

"Amongst the many gifts Doctor Who has given me, to end up being the Brigadier’s grandfather, I couldn’t have imagined anything more brilliant," he said.

You can read more about this callback here.

9. “Here we go – the long way round”

The First Doctor’s final words in the episode play double duty here, with the “here we go” comment echoing a sentiment made by Neve McIntosh’s Vastra during Peter Capaldi’s first Doctor Who episode (she said “Here we go again” in reference to the Doctor’s latest regeneration) and the “long way round” comment calling back to 2013 50^th anniversary special The Day of the Doctor and other episodes.

In the Day of the Doctor, it’s used to indicate the Doctor’s new quest to seek out Gallifrey, and repeated in 2015 episode Heaven Sent by Capaldi’s Doctor and that year’s finale Hell Bent by Jenna Coleman’s Clara, both referring to imminent trips to Gallifrey. You can see the quotes below.

The Day of the Doctor

DOCTOR [OC]: Clara sometimes asks me if I dream. Of course I dream, I tell her. Everybody dreams. But what do you dream about, she'll ask. The same thing everybody dreams about, I tell her. I dream about where I'm going. She always laughs at that. But you're not going anywhere, you're just wandering about.

(He walks out to join his past selves, backs to us, gazing out at the stars.)

DOCTOR [OC]: That's not true. Not any more. I have a new destination. My journey is the same as yours, the same as anyones. It's taken me so many years, so many lifetimes, but at last I know where I'm going.

(A big golden planet hangs in the sky. He stands between the 10th and 8.5 Doctors.)

DOCTOR [OC]: Where I've always been going. Home, the long way round.

Heaven Sent

DOCTOR: Go to the city. Find somebody important. Tell them I'm back. Tell them, I know what they did, and I'm on my way. And if they ask you who I am, tell them I came the long way round.

Hell Bent

GENERAL: Gallifrey is currently positioned at the extreme end of the time continuum, for its own protection. We're at the end of the universe, give or take a star system.
DOCTOR: I know. I came the long way round.

…

ASHILDR: Where are we going?
CLARA: Gallifrey. Like I said, Gallifrey. The long way round.

10. The regeneration speech

Capaldi’s own farewell speech contains all sorts of hidden messages, including a running gag about pears, a “be kind” message repeated from series 10 finale The Doctor Falls and a theory about the Doctor’s name that Capaldi himself came up with several months ago.

We’ve assembled all these speech Easter Eggs in one handy article for you elsewhere, because frankly this article is getting a little too big on the inside.

11. Musical moments

Longtime Who composer Murray Gold is leaving the series after this episode following over 12 years of scoring every episode – so it’s no surprise that he snuck in a few of his old hits, with the original ‘Doctor theme’ introduced in 2005 and 2006’s fan-favourite piece ‘Doomsday’ also audible at one point in the new episode.

Clara's theme from later seasons also appears when Jenna Coleman's companion makes her cameo (see below).

12. Smacked bottoms

The First Doctor’s threat to smack companion Bill’s (Pearl Mackie) bottom for her bad language is actually taken fairly directly from a comment made by Hartnell’s Doctor in 1964’s The Dalek Invasion of Earth, where the Doctor tells Susan (Carole Ann Ford) “what you need is a jolly good smacked bottom!”

13. The Doctor's past (and future)

During an early face-to-glass meeting with Testimony, the Doctors are treated to something of a clipshow from the Doctor’s past (or, in the First Doctor’s case, his future), featuring footage of various Doctors and episodes from Doctor Who over the years.

Doctors easily visible in the display are Tom Baker, Peter Davison, Jon Pertwee, Paul McGann (in 2013 short episode The Night of the Doctor), Matt Smith, David Tennant, Capaldi himself and most prominently the late John Hurt, whose War Doctor most closely matches the description of the ‘Doctor of War’ Testimony are trying to demonstrate.

Footage and audio from episodes including The Parting of the Ways, Journey’s End, The Day of the Doctor and Utopia and various classic serials is also used in the floating bubbles.

14. The Names of the Doctor

Apart from the Doctor of War (which is, as noted above, a play on Jon Hurt’s War Doctor), the Time Lord receives a few more nicknames from Testimony in this sequence, most of which correlate to earlier stories. Audio of Julian Bleach’s Davros naming him The Destroyer of Worlds (from 2008’s Journey’s End) can be heard, along with David Tennant’s Doctor describing himself as The Oncoming Storm – a nickname the Daleks supposedly had for the Doctor which has been referenced multiple times in the new series.

Steven Moffat’s first series finale The Pandorica Opens/ The Big Bang is also called back to by the Testimony’s description of the Doctor as “The Imp of the Pandorica,” while Matt Smith’s final adventures in The Name of The Doctor and The Time of the Doctor are paid tribute to with the name “The Beast of Trenzalore” (Trenzlore being the Doctor’s mythical final resting place and the main setting for those episodes).

“The Destroyer of Skaro” name the Doctor is given here is pretty self-explanatory, given the number of times the Doctor has directly or indirectly destroyed the Dalek homeworld over the years, while “The Butcher of Skull Moon” is a slightly more obscure one, referring to a battle in the Time War the Doctor was revealed to be a part of in 2015 finale Hell Bent.

And FINALLY, the Testimony calling the Doctor “The Shadow of the Valeyard” refers back to classic Colin Baker serial The Trial of a Time Lord, where a character going by that name (played by Michael Jayston) is revealed to be an amalgamation of the Doctor’s darker sides and a possible future incarnation.

15. Return for Rusty

One of the more surprising returns from the series past came in the form of conflicted Dalek Rusty, who previously appeared in 2014 episode Into the Dalek when Capaldi’s Doctor and Jenna Coleman’s Clara shrunk down to find out what was making him different to his homicidal brethren.

You can read more about Rusty’s return here.

16. Even more cameos

Of course, Rusty wasn’t the biggest cameo in the episode, with both Matt Lucas’ Nardole and Jenna Coleman’s longtime companion Clara Oswald returning for some final goodbyes with the Twelfth Doctor.

Clara’s appearance resolves a couple of plotlines introduced in 2015 finale Hell Bent (the Doctor’s memory was erased in that episode, and is restored in Twice Upon a Time), and according to writer Steven Moffat was filmed much later than the rest of the episode due to Coleman’s busy schedule in ITV’s Victoria.

17. And finally...those unreliable Tardis windows

When David Bradley’s First Doctor encounters Peter Capaldi’s Tardis, he immediately remarks that the windows are the wrong size, an observation echoed by companion Bill (Pearl Mackie) when she observes Bradley’s earlier version of the time machine.

This is a bit of an in-joke among certain Doctor Who fans who have long complained about the Tardis’ incorrect dimensions in comparison to real police boxes, and was previously teased in perhaps Steven Moffat’s most famous episode to date – 2007’s critically-acclaimed Blink, still named as most fans’ favourite Who episode to this day.

“This isn't a real one,” Michael Obiara’s Billy Shipton tells Carey Mulligan’s Sally Sparrow of the Tardis. “The phone's just a dummy, and the windows are the wrong size.”

Clearly, departing showrunner Steven Moffat couldn’t resist a quick tour of his greatest hits.

Doctor Who returns to BBC1 in autumn 2018