(Photo: BBC America) 
Never mind the “what? What? WHAT?,” these are the geekiest moments from the Thirteenth Doctor’s swansong, the bits that need a little bit of explanation for anyone not entirely au fait with the last 60-odd years of Doctor Who, the TV series.  
Let us start at the very beginning: 
1. The Title 
“The Power of the Doctor” is a throwback to the very first post-regeneration story in Doctor Who history. “The Power of the Daleks” was Patrick Troughton’s first adventure, and saw him take the helm of the TARDIS as the Second Doctor. Interestingly, it is also the only other story in Doctor Who history in which the Doctor’s clothes regenerate as well as their face/body. Notice how David Tennant as the Fourteenth Doctor is wearing his own stuff? That’s “Power” power, baby! 
2. The Recorder 
While being the Doctor, the Master plucks a blue and white striped recorder from his pocket and plays “The Skye Boat Song.” This is another tip of the hat to the Second Doctor, as it was his habit to play that very instrument and indeed, that very song in “The Web of Fear”.   
3. “The blossomiest blossom” 

 

In “The Time Monster,” the Third Doctor tries to cheer up a despondent Jo Grant with a story about how he visited a monk as a depressed young boy, and asked him for the secret of life. A withered hand pointed to a feeble flower on the floor, as the Doctor recalled, “I looked at it for a moment and suddenly I saw it through his eyes. It was simply glowing with life, like a perfectly cut jewel. And the colors? Well, the colors were deeper and richer than you could possibly imagine. Yes, that was the daisiest daisy I'd ever seen.”  
4. The Picture 
Let’s face it, when they get together a group of former TARDIS travelers in a room to talk about their time with the Doctor, and one of the chairs has a framed photograph on it, there’s really only one person whose face fans are expecting to see. Even though we never see it, and it could be anyone from Barbara to Polly to Victoria to Liz Shaw, or even the companions that never made it home - Adric or Clara – the person whose face is undoubtedly in that frame is Sarah Jane Smith.  
5. Adric 

 

The Doctor’s hologram, pausing to look somewhat like the Fifth Doctor, has a conversation with Tegan in which she asks him what she must be feeling seeing Cybermen again, and he replies “Adric.”  This is more than a geeky spot, this is unfinished business, in that Tegan was present when the Doctor’s friend Adric died at the helm of a spaceship crashing into prehistoric Earth, thereby wiping out the dinosaurs. It does make the Master’s plan of sending her the Lone Cyberman in shrunken form seem particularly cruel.  
6. Respect Your Elders Part 1 
Hats off to William Russell, who played Ian Chesterton, the First Doctor’s first human travelling companion – with teaching friend and colleague Barbara Wright. Aged 95 at the time of his appearance in Graham’s group, he’s the oldest actor to appear onscreen in the revived Doctor Who.  
7. Respect Your Elders Part 2 

  

The second oldest (so far) is Ysanne Churchman, whose squeaky voice is recognisable as Alpha Centauri at the end of “Empress of Mars”. She was 92 at the time of filming.  
8. Melanie Bush 
Here’s a fun fact; the companion in Graham’s group who says, “How many Doctors are there?” is Mel, played by Bonnie Langford. In the TV show, she was never given a surname. To rectify this, the makers of Doctor Who spin-off stories in both audio and prose elected to give her the last name Bush, and this has now been formally made canon within the TV show for the first time.  
 9. Croydon
Yaz admits that she could only pilot the TARDIS well enough to drop everyone off in Croydon. This is Sarah Jane Smith’s home territory, and therefore means that Yaz is officially better at driving than the Doctor. Her Fourth incarnation attempted the same tricky landing to drop Sarah home, and could only manage Aberdeen, nearly 550 miles away.   
10. “Your dad was an idiot” 
Time for some primo fan-service taunting by the Master. First he glowers at Kate Stewart, that her dad was an “idiot,” a not-particularly-damning slur on her father, the Third Doctor’s fondest bickering partner, Brigadier Alasdair Lethbridge Stewart.  
11. “How's your Auntie Vanessa? Do you keep her in a little doll's house?” 
On a more barbed note, this jibe, aimed at Tegan Jovanka, is a reference to the fact that a previous incarnation of the Master killed her aunt Vanessa with his tissue compression eliminator after she entered his TARDIS, thinking it was an actual phone box (“Logopolis,” another regeneration story, this time for the Fourth Doctor).  
12. “Didn't the Doctor ditch you? No? Little fallout with your Machiavellian maestro?” 
And then there’s Ace. This barb references the end of the Seventh Doctor’s relationship with Ace, something we don’t ever see on screen. It was depicted by Sophie Aldred (Ace herself) in the novel At Childhood's End. Oddly, the novel suggests that Ace – known as  Dorothy McShane – has met the Thirteenth Doctor, Yaz, Graham and Ryan before.  
13. “Ra Ra Rasputin” 

 

As if the sight of the Master dancing to the exceptional 1979 disco hit “Rasputin” by Boney M wasn’t odd enough, it actually has some backstory in the world of Doctor Who. Back in the days of the Twelfth Doctor, Peter Harness, writer of “The Zygon Invasion” / “The Zygon Inversion” suggested a story called “How the Monk Got His Habit”.  
In this tale, the Meddling Monk (a proto-Time Lord character last seen battling the First Doctor in “The Time Meddler”) launches an eccentric plan to go back to 1916 with some kind of media player, and let the actual Gregori Rasputin hear Boney M’s masterwork. Things go awry, and the Monk ends up being ordered by the Twelfth Doctor to take Rasputin’s place for the rest of his natural life.   
If you had any lingering questions about the special, have they all been answered?