MI6 Agent O manages to keep several of the beings of light at bay with a forcefield he's cribbed together for his shack in the Australian Outback. Of course, somewhat subverted after the reveal at the end of the episode.
O turns out to be none other than the most recent incarnation of the Master who has, once again, given the middle finger to death, a fact he kept from the Doctor for the vast majority of the episode. Upon revealing this, he proceeds to have one of his goons trap the Doctor in the alien dimension and strand the companions on a crashing plane that's just had its cockpit blown away by a bomb he planted.
The Doctor manages to catch "O" in a statement that contradicts his background story, and points this out to him. "O" freezes for a couple of seconds, and you can see the gears turning in his head as he considers whether he should try to keep up the charade with a quick lie or just drop it altogether. He settles on telling the Doctor what is going on, clearly because he thinks it is the funniest and most satisfying option. Even the way he reveals it, detailing how he was hidden in plain sight the entire time.
O: Doctor, I did say "look for the spymaster". Or should I say... Spy-Master?
Sacha Dhawan especially deserves credit for his acting here during the reveal. With just one line, his whole personality shifts and he just becomes The Master, even channeling each of his predecessors in quick succession.
Ada Gordon takes advantage of the Master being distracted by lording it over the Doctor to force him to flee the room by attacking him with a machine gun and grenades that were on display at the invention fair everyone is at.
The Doctor, in a Bond-style story, gets the Master to engage in Bond Villain Stupidity, orchestrating an Engineered Public Confession she records for the Kasaavin to hear, while also trapping him in the past with the Nazis and stealing his TARDIS.
The Daleks spent ages trying to annihilate the Time Lords over the course of the Time War. After finding out the truth of the Timeless Child, the Master just went and did it. And considering the Master once obliterated a quarter of the universe by accident, you know that they're the only Time Lord who's capable of wiping them out in one fell swoop. And considering how terribly the Time Lords have treated the Master and the Doctor even after they've been saved by the both of them a bunch of times, it's not undeserved.
The Doctor teams up with Nikola Tesla to use Wardenclyffe tower to shoot a beam of lightning at the Skithra ship. The plan almost fails, but the Doctor tricks the Skithra Queen into taking her teleport bracelet so she can force her back onto the ship. Unfortunately, the Skithra ship appears to escape, but it seems to have been damaged.
The Skithra, it turns out, have stolen all their technology (not unlike Edison), and need a brilliant mind like Tesla to fix it. The Doctor gives a ruthless warning worthy of Seven or even Ten, showing the Skithra really have no idea who they're messing with.
The Doctor: When you die, there'll be nothing left behind. Just a trail of blood, and other people's brilliance. No one will even know you existed. Skithra Queen: Ha! Tell me, Doctor... have you ever seen a dead planet? The Doctor: ...I've seen more than you can possibly imagine.
The return of Captain Jack Harkness after 9 years.* Or 10, depending on whether you count from "The End of Time" or Torchwood: Miracle Day. The build-up to the reveal plus the Leaning on the Fourth Wall moments had the majority of the fandom screaming with excitement:
Jack Harkness: HA! You missed me, right?
Ruth unleashing on a squad of Judoon unarmed, breaking off the captain's horn, forcing them to teleport back to their ship, while shouting some aggressive Judoon-ese.
Ruth's return to her true form. While where she stands in the Doctor's timeline is in doubt, there is no doubt that seeing her as the Doctor, armed and ready to fight back, is a powerful moment.
Ruth: Let me take it from the top. Hello. I'm the Doctor.
Jake, an off duty police officer, flies a whole spaceship with no experience, to show his astronaut husband that he's not scared of living. In doing so, he deposits the cure to the eponymous virus throughout the jetstreams, all the while knowing the ship is about to blow. Which is followed up immediately by the Doctor materializing the TARDIS around the exploding ship, saving Jake at the last possible moment. The Big Damn Kiss Adam gives Jake just seals the deal.
When confronted with choosing between sacrificing Percy Shelley, thus seriously screwing up Earth's timeline post-19th century, and letting the Lone Cyberman have what it wants, thus causing the rise of the Cyberman empire and the destruction of humanity billions of years into the future, the Doctor ends up Taking a Third Option by using an old Time Lord trick to bring Shelley's mind to the time of his death to trick his body into momentarily thinking it is dying, thus forcing the Cyberium out. Doubles as Nightmare Fuel since said "Time Lord trick" is pretty horrifying.
After a few lifetimes worth of Trauma Conga Line, and admitting she's so tired, the Doctor using all her memories (with footage spanning from Capaldi to Hartnell), including the painful ones, to overload the Matrix and rescue herself. The remixed theme tune playing all the while just adds to the awesome.
The companions all escape from danger completely without the Doctor's help, and go to her rescue to boot.
The Doctor giving the Master a Shut Up, Hannibal! speech about how the new knowledge of being the Timeless Child has not broken her, but it was a gift, and that she isn't scared of having more history than she knew like he would be. The way his face just falls only adds to the scene.
The Doctor: You think you've broken me? You'll have to try harder than that. You've given me a gift. Of myself. You think that could destroy me? You think that makes me lesser? It makes me more. I contain multitudes more than I ever thought or knew. You want me to be scared of it because you're scared of everything. But I am so much more than you.
Whatever prison the Doctor has been put in, it thoroughly subverts the Cardboard Prison trope. The Doctor is able to waltz past a Weeping Angel and a Pting (which very well may be the same one fought earlier) tries and fails to eat the cell.
Jack rescuing her from there is no small feat either, as he admits it took 19 years in-universe for him to get a cell next to the Doctor, and they escape using a matter-phasing device.
The Doctor pulling one hell of a Batman Gambit (as much as Jack rightly points out it's a stupid plan). She beacons a fleet of Death Squad Daleks to Earth, knowing they will attack even the other Daleks who they see as impure
And a further one in tricking the Death Squad into the spare TARDIS the fam acquired earlier in the series and compressing it around them, while constantly insulting them.
As much as she is a Corrupt Politician, the Prime Minister in the episode point blank refuses to work with the Daleks. (And gets exterminated on the spot)
