The War Doctor has run out of options to save the universe. To keep all of existence from burning, he has to destroy both Gallifrey and the Daleks. To this end, he steals a weapon called the Moment. It is a weapon so destructive that the Time Lords kept it locked away, daring not to use it themselves. The Moment developed a consciousness at some point, and presents itself to the War Doctor as Rose Tyler's "Bad Wolf" incarnation. She – or it – decides to show him what he will become in the future, so that he can make an informed decision about committing murder on such a massive scale.
The Moment opens portals to future Doctors that, sadly, skip over Nine, but find their way to Ten and Eleven. (I'm going to call them by numbers. It seems the easiest way.) Ten is pretending to court Queen Elizabeth I in order to prove she's a Zygon duplicate, while Eleven is about to embark on an adventure with Clara when UNIT brings the TARDIS to the National Gallery. Kate Lethbridge-Stewart has a letter and a painting from Elizabeth I. The letter, from 1562, tells of the dire Zygon problem, while the painting is meant to serve as her credentials, proving that the letter is genuine. It's a work of Time Lord art, capturing a moment in time, right before Gallifrey's total annihilation. It's titled either "No More" or "Gallifrey Falls" – nobody knows for sure.
Eleven zips back to 1562, where he meets up with both Ten and the War Doctor, thanks to those portals. There's not much love lost between all the Doctors. The younger one thinks his future selves are a bit showy and ridiculous, while his future selves would rather leave him buried deep in their past. While trying to determine which Elizabeth is real and which is an imposter, they find themselves captured by Zygons and locked in the Tower of London. Luckily, that's just where Kate's future office is, so Eleven is able to leave her a message that lets Clara pop back and save them. It turns out she didn't actually need to save them, as the door wasn't even locked, but it gives the Doctors a chance to realize how powerful their combined minds (and sonic screwdrivers!) can be.
Meanwhile, in 2013, the Zygons have used that painting and many other works of Time Lord art to infiltrate the National Gallery. They have a device that allows them to freeze themselves within the three-dimensional framework of the painting. When the time is right, they smash their way out and try to take over the Earth. They impersonate Kate and her assistants in order to gain control of the Black Archive beneath the Tower. Since the Archive is TARDIS-proof, the assorted Doctors use this same device to freeze themselves in the painting and return to 2013 just in time to thwart the Zygons. They do this by temporarily wiping everybody's memories inside the Black Archive so that the imposters and their originals are indistinguishable from one another. They literally can't remember who's human and who's not. From this even playing field, they're able to broker a peace treaty that's completely fair and binding.
In the end, Clara and the Doctors return to the point where the War Doctor was about to deploy the Moment. The Doctors are more understanding of their earlier self, and even reach out to help him push that final button. A word from Clara gives them pause, however, and that pause gives them enough time realize (again) how well they work together. They have another option! Inspired by the Zygon freezing tech, they program every TARDIS to do something similar, then surround Gallifrey and freeze it inside a pocket universe. To the main universe, it will look as though Gallifrey was destroyed. The Daleks will end up blasting each other out of the sky when their target suddenly vanishes out from between them. Sadly, the War Doctor will never remember this because of that whole timey-wimey thing, and probably begins his regeneration thinking he's a murderer. Ten, too, will forget and go back to believing in his own mass-murdering history.
But Eleven remembers the truth, and Twelve (seen only from the eyes up in a brief flash) will remember. When Eleven returns to the National Gallery to ponder the origins of that painting, a kindly old Tom Baker (!!) fills him in on a little detail. The painting doesn't have two names at all. It's not "No More" or "Gallifrey Falls," but rather "Gallifrey Falls No More." Eleven takes that as a sign that his/their scheme worked and Gallifrey is out there, somewhere, waiting for him – or some version of him – to rediscover. Stay tuned for the full weecap.
The old-school, original theme music and title card transport us back 50 years to the onscreen birth of Doctor Who. Happy birthday, Doc! After that, we find ourselves in the present day, at the secondary school where Clara is teaching a gaggle of children about Marcus Aurelius. The moment the bell rings, a young man rushes into her classroom to tell her a doctor has left a message for her. He's quite worried something is wrong with her, but Clara knows it's the capital-D Doctor. She hops onto her motorbike and heads to the appointed address, some miles away in the middle of a deserted country road. She bikes her way right into the TARDIS, then shuts the doors with a snap of her fingers. They make plans to visit ancient Mesopotamia, then hug in greeting, only to be interrupted when a giant claw drops down from a helicopter and snatches up the TARDIS.
Action movie music plays, like it's some kind of major trouble happening, but it turns out to be a misunderstanding. The Doctor calls in to UNIT headquarters, where an assistant in a mini version of Four's scarf hurries to hand the phone to Kate Lethbridge-Stewart. The assistant is so out of breath with excitement that she has to take a hit off her rescue inhaler. "Oh, my God, Doctor! We had no idea anyone was in there," Kate says when she hears the helicopter on the other end of the line. "Next time, would it kill you to knock?" shouts the Doctor as he dangles precariously over the city.
The helicopter deposits the TARDIS and its occupants outside the National Gallery. Kate apologizes heartily for the mix-up. "I'm acting on instructions straight from the throne," she explains. She hands over a small note sealed with red wax. "Sealed orders from Her Majesty, Queen Elizabeth I." She then brings them all inside to show them the Queen's credentials.
On their way in, Clara tries to get the Doctor to tell her about how he knows Elizabeth I. He changes the subject to talk about UNIT. "They investigate anything alien," he says. "What, like you?" she asks. "I work for them," he says. Clara is surprised to learn he has a job. The conversation is cut short when they reach Her Majesty's credentials, which take the form of a large, gold-frame painting... of sorts. It moves and shimmers in three dimensions. "No More," the Doctor murmurs. "That's the title," Kate clarifies. "Also known as Gallifrey Falls." It depicts the fall of Arcadia, Gallifrey's Second City, he explains, in a slice of real time. Like the TARDIS, the painting is bigger on the inside. "Elizabeth told us where to find it, and its significance," Kate says. "He was there," the Doctor whispers. "Me. The one I don't talk about." It's the incarnation he's tried to forget. "He was the Doctor who fought in the Time War and that was the day he did it. The day he killed them all. The war to end all wars, between my people and the Daleks."