There are a lot of assumptions fans make about Doctor Who. The Doctor always has a plan, and when put into practice, people get saved. The companion, or companions, survive every episode, enclosed in plot armor as thick as the Doctor's. If a companion or the Doctor leaves, fans usually know well ahead, as recasting these roles can be a long and arduous process. So perhaps the most effective thing anyone involved with the show can do is yank the rug out from under those certainties.
The Doctor: I used to hotwire warp drives for fun on weekends as a teenager. Not that we had weekends or teenagers. Basically, I used to do this a lot and people got mad.
That's what the episode does within the first ten minutes. After an opening proclaiming the imminent return of the Cybermen, the show cuts to somewhere in the past, on Earth, as a young Irishman, Patrick (Branwell Donaghey), who finds an abandoned baby. He and his wife, Meg (Orla O'Rourke), take the child in, after the local Gardaí, Michael (Andrew Macklin) can't find the parents. As fans try to work out how the child, Brendon, figures in, the action cuts to the future, where the last humans, mostly nurses, teachers, and children, attempt to outrun the Cybermen.
But never fear, the Doctor is here! Graham has a device to turn the Cybermen's emotions on, Ryan has a forcefield, and Yaz's machine radiates gold particles (Cybermen are allergic). And none of it works.
Cyberdrones (flying Cybermen heads) take out the machines like the flimsy props they are. Half the refugees die immediately, including Feekat (Steve Toussant) and Fuskle (Jack Osborn). Genuinely terrified in a way we've not seen, the Doctor orders her companions to leave with the refugees. But Ryan gets separated, and only Yaz and Graham wind up with the survivors: Ravio (Julie Graham), Yedlarmi (Alex Austin), and Bescot (Rhiannon Clements). Ryan and refugee Ethan (Matt Carver) wind up left behind.
The Doctor steals a Cyberfighter with Ryan and Ethan to get off the planet. The goal, according to the refugees, is to get to Ko Sharmus, where they will cross "The Boundary" to safety. On the refugee ship, Yaz relishes the opportunity to be the driving force when it breaks down. Graham also cheerleads, needling those among the crew, like Yedlarmi, who seem ready to lay down and die. Their ingenuity gets the team onto an old Cyberman war carrier, left for rubbish, which can get them to Ko Sharmus, as soon as they can power it up. But Team TARDIS' plans failed once already. It's an open question of who will survive this adventure. After all, Jodie Whittaker is confirmed to return for Season 13, but the rest of the cast is not.
Ryan, Ethan, and the Doctor are slightly luckier in that the Cyberfighter has the capability for a nonstop flight to Ko Sharmus. But as they travel, the Lone Cyberman sends a message, once again speaking in a religious fever of how he is the chosen one, destined to bring forth the Cyberarmies once again. (It's why he was rejected from being a Cyberman initially. Or something like that.) The Doctor does the usual "Look me up and be afraid!" bravado speech, but the Lone Cyberman is not impressed. He's looked her up, and she's destined to die along with everyone else.
As for now-adult Brendon (Evan McCabe) in 20th century Ireland, he's joined An Garda Síochána, the Irish police. One day, not too long after getting hired, he chases a thief to the cliffs, just outside of town. In desperation, the robber shoots Brendon in the chest, sending him falling hundreds of feet to his death. Or what should be his death — as his now-boss Michael looks on, Brendon wakes, unharmed, a miracle made flesh. Mama Megs is delighted at her son's luck and the story in the papers about his survival. But Patrick and Michael exchange silent glances, and never look at Brandon the same way again.
But that's not the only odd thing that happens to Brendon. The decades pass, he grows old, but the village seems to be stuck in the 1940s. It's not the only thing that hasn't changed. Brendon is old and gray, retiring from the force after a long and faithful service, but Patrick and Michael still look as they did the day he came home in a basket 50+ years ago. As Brendon leaves the Garda one last time, they're waiting for him. They turn him around and march into the back office. What the device is they hook him to is unclear (though one assumes Cybermen-related). But their purpose is not. Brendon's done good work for them. Too bad when they're done, he won't remember any of it.
The Doctor reaches Ko Sharmus, only to discover it's not a place but a man. Sharmus (Ian McElhinney) has sacrificed his life as the Guardian of The Boundary, a wormhole-like crossing on the edge of the sea, hoping to usher through the last remaining humans should they come. The Doctor, Ethan, and Ryan have made it, but Yaz, Graham, and the refugees might not be so lucky. The war carrier was carrying troops, thousands upon thousands of fresh Cybermen, still in stasis. With the Lone Cyberman on board, ready to activate the whole lot, it's not long before the last remaining humans find themselves having reached Ko Sharmus, but unable to disembark, holed up on the bridge of the ship with thousands of Cybermen breaking down the door.
With no way out and the fear the show is finally ready to pare down the Doctor's ensemble of helpers, this cliffhanger would already be one of the best Doctor Who's aired in years. But it's not finished. As the Doctor stares into The Boundary, the place on the other side becomes apparent. It's Gallifrey, still lying in ruins as she saw it in the season premiere. As she tries to figure out why that's where The Boundary goes, a figure pops through crossing to their side. It's the Master, and he's here to tell the Doctor to be very afraid indeed.