This week's Doctor Who begins in Gloucester. Ruth Clayton (Jo Martin), a 44-year-old married woman, just woke up on her birthday. It's a typical workday for her, trying to convince tourists to take her tour, while the local barista, Allan (Michael Begley), tries to persuade her to go on a date. It's a bit odd, since she's married, but Allan has a whole dossier on her husband, Lee (Neil Stuke), claiming he's not who he says he is. Ruth laughs his sad attempts at courtship off. But today is about to prove Allan's on to something.
Doctor: A platoon of Judoon near the moon? They can't do that.
It is a platoon of Judoon, and they've arrived near the lagoon by the Cathedral, where Ruth stations herself every day. Their arrival on Earth signals the Doctor in the TARDIS, distracting her from her secret continuing quest to find the Master and ask him more questions about what happened to Gallifrey. But Space rhinos are a trigger-happy lot, and before the Doctor can even get there, the Judoon have killed Ruth's friend Marcia (Judith Street) for her knitting weapon, and are marking all the humans, as they search out "a fugitive."
That fugitive seems to be Lee, who rounds up Ruth to take her home, without admitting what's going on. Luckily, the Doctor and fam arrive in the city by landing in the coffee shop where Allan works, just as Lee runs out. Allan's soon pointing the Judoon towards Lee, and the Doctor and company follow. Well, all except for Graham, who disappears in a flash of blue light. Confronted by the Doctor, Lee seems to admit it's him the Judoon are after, and begs her to get Ruth out of danger. Ryan and Yaz declare they'll help distract the Judoon so Ruth and the Doctor can run, but they too disappear in a flash of blue light once downstairs.
Meanwhile, the Doctor takes Ruth to hide in the Cathedral, at Lee's suggestion. There, Ruth gets a text from Lee that triggers a memory: "Follow the Light. Break the Glass. Happy Birthday. X." While she and the Doctor run, Gat (Ritu Arya), the representative of the end client, beams down to have a chat with Lee. But it turns out he's not the fugitive, which is why she cheerfully shoots him dead. Lee was the cover; Ruth is the real target. The Judoon teleport to the Cathedral, back on the hunt. Ruth is dizzy from her triggered memory, with no idea why these people are after her. But the message awoke something because she not only beats the Judoon in a fight, but she knows how to humiliate their leader by snapping off his horn and sends them packing.
And that's when things start to get complicated.
Let's start with what happened to Graham. He got scooped by a stolen ship looking for those with traces of time travel and evidence of recent time in the TARDIS. It's Captain Jack Harkness (John Barrowman), and he's looking for the Doctor. (Sadly, Ianto doesn't seem to be with him, nor anyone else from Torchwood.) Exactly where Jack is coming from or where he's headed isn't explained, but he grabbed Graham assuming, like everyone else, that the oldest male of the group is the Doctor. But rather than be annoyed at his mistake, Jack is super excited when he hears the Doctor is now a woman.
The problem with stolen property is that it doesn't behave right, and worse, the Judoon containment field is screwing with the sensors. Jack winds up managing to scoop everyone except the Doctor, meeting all three companions. (Three? Oh, he's dreamed about this.) Ryan and Yaz take a shine to the guy, even though they can't help but note he's cheesy as hell. (Good cheesy, though!) Unfortunately, Jack then discovers he's being attacked by the ship's nanogenes, an anti-theft device. He needs to exit in a hurry, so he leaves them with a message for the Doc: "Beware the lone Cyberman."
The companions have not met cybermen yet, and Ryan asks what they are. Jack calls them an "empire of evil," tells them the Cybermen are currently defeated, reduced to nothing. But that could change, if the Doctor does not heed his words: "I'm going to see her again. Maybe not soon, but if she needs me, I'll be there... Beware the lone Cyberman. To defeat them, the Alliance sent this thing back through time, across space.... The lone Cyberman, don't give it what it wants, at all costs." He then returns the companions to Earth and heads out.
Meanwhile, the Doctor has pushed Ruth to head towards the place the text triggered a memory of — a lighthouse, where she remembers growing up with her parents, and senses "nothing can hurt me here." Once arrived, the Doctor starts exploring. Out front, there's an unmarked grave, and on a hunch, she starts digging. Meanwhile, Ruth enters the kitchen, where she sees a small metal dial attached to the wall. It looks like a gas meter, but it's actually one of those "In case of emergency, break glass" things, just far too small to be of use. Driven by the voices in her head, Ruth breaks the glass.
Whoheads will not be surprised to see what looks just like regeneration energy flows out of the circle, into Ruth. The dial was a chameleon arch, technically a device with which a Time Lord can disguise themselves to infiltrate primitive societies without any danger of interfering with the Web of Time because it erases the memory of who they are. In practice, the Doctor and the Master have both used these to hide from persuers. In the Doctor's case, Martha was his protector, much like Lee was to Ruth. Ruth is a Time Lord.
But not just any Time Lord. She's the Doctor. Our Doctor. Just a different regeneration, one we haven't met before.
Her TARDIS is buried under that gravestone. It's the very same police box every Doctor has flown, but full throwback inside, and she has no memory of ever being Thirteen. Moreover, when she and Gat (who is also a Time Lord) confront each other, it is obvious Gallifrey still stands. She is a past Doctor.
Except, the Doctor was never a black woman. Gat never chased her. (She certainly never rigged a laser gun so Gat would kill herself if she fired it.) This adventure never happened. So, what in heaven's name is going on? Even without an explanation, Chibnall has gone full Moffat, adding another Doctor to the chronology. We don't know where she falls, or who she was. But the end credits confirm: "Introducing Jo Martin as The Doctor." She's the real deal, and all timelines, everywhere, are canon. Even if this is undone, at some point, in some timeline, there is now a confirmed black woman Doctor. (And oh my, do I love her outfit.) This is a historic day.
Once free of the Judoon, The Doctor offers the Doctor a ride back to her TARDIS, still parked in the Gloucester coffee shop. But she does not take kindly to The Doctor having an attitude over her methods, and The Doctors part on less than cordial terms, both emotionally shaken by their run-in. Once reunited with her fam, The Doctor decides to come clean, explaining that whatever just happened, things are about to get real around here. Despite the Doctor feeling every one of her thousands of years and her slightly bitter attitude towards humans who claim they can help, Ryan, Yaz, and Graham stand by her, reminding the Doctor of what's important. She's right that they don't know who she was in the past or the future. But she's their Doctor, here and now and they are here for here, as long as she needs them.