The one with the bookended Big Bad.
On a planet, an older woman and a younger man arrive at a small lake. The woman, Andinio, decrees that this is the place, but the man, Delph, is unsure, telling her that her teachings didn't make him very sure of his place in the universe. She reassures him, and tells him to do what only they can do. His eyes glow golden, and he begins pulling rocks from the surrounding area and bringing them together above the water. Andinio notices something odd, and tells Delph to stop as something appears behind them, prompting Andinio to gasp that it can't be.
3,407 years later, the Doctor and her companions pick up no less than nine distress signals coming from the planet Ranskoor Av Kolos, which the Doctor notes translates as "Disintegrator of Souls", prompting Graham to snark about how cheerful that sounds. Before they head off to see if they can help, the Doctor gives everyone neural stabilizers that will keep them safe from the mind-altering influences of the atmosphere. The TARDIS lands on a parked spaceship, where they find a confused man with no memory and a mysterious, impossible crystal. After he starts to remember things once the Doctor gives him a neural stabilizer, it transpires that his name is Paltraki and that he and his crew came to this planet for a reason. The ship is contacted by Andinio, who demands that Paltraki return the "object". She is followed by a masked Stenza warrior who murders one of Paltraki's remaining crewmembers, who are hostages, telling him that he has until lightfall to return what he stole or the other two die. The Doctor and her friends are shaken, wondering if the Stenza warrior is, in fact, the long-vanquished Tim Shawnote .
Gathering gear, they set out to rescue Paltraki's crew. Graham pulls the Doctor aside and tells her that if the Stenza is Tim Shaw, he's going to kill him. The Doctor warns him against doing that, telling him that he's better than that. Their destination is a mysterious floating building, surrounded by a heap of crashed starships. Paltraki remembers that he and his crew were sent to get something from that building. When they reach the edge of the lake beneath it, the Doctor distributes gear. Ryan asks her why she's brought grenades if she's so against weapons, and she tells him that destroying things that can be rebuilt is different from using the weapons on people. The Doctor tasks Ryan and Graham with freeing Paltraki's crew, and Yaz and Paltraki with finding out where the crystal came from. She herself heads off to deal with the Stenza. Sorted, they all teleport into the building and split up.
Graham and Ryan, after fleeing from sniperbots, discover not two but dozens of people in stasis chambers. Yaz and Paltraki discover a room with several more crystals just like the first, and Paltraki remembers that his mission was to retrieve those crystals. The Doctor, meanwhile, runs into Andinio and recognizes her as one of the Ux, an incredibly rare Reality Warping species only found on three planets, and only consisting of two members at a time. It turns out that the Stenza is, in fact, Tim Shaw, who wound up on Ranskoor Av Kolos after his recall device was somehow altered. The Ux, or at least Andinio, mistook him for their creator god, which he abused to the extent that he now believes himself to really be a god. He's had the Ux create and power a superweapon, and he's having Andinio start it up right now. Delph has spent a rather long time chained up as a power source, since he harbours doubts about Tim Shaw's alleged divinity. It turns out that the crystals contain captured and stolen planets, including five of the Nine Planets that Paltraki and his crew come from.
The Doctor is horrified, since that many shrunken planets, with their mass suspended by technology that doesn't make sense, could destroy the universe. As Graham and Ryan work to free the prisoners and Paltraki goes to help them, the Doctor tries to persuade Andinio to stop what she's doing, as Tim Shaw has decided since he wants revenge upon the Doctor that the next target of his superweapon should be Earth, the site of their first meeting. Eventually, the Doctor and Yaz use their neural stabilizers to cut off the Ux's powers, stopping them in their tracks. The Doctor manages to break Andinio out of believing that Tim Shaw is the Creator and, trying to figure out how to get the planets back to their proper locations before their containment crystals fail, summons the TARDIS to her. Combining the TARDIS' telepathic circuits, both Ux's powers, and the Stenza technology allows all the planets to be restored to their proper places. Meanwhile, Tim Shaw discovers that Ryan and Graham are releasing his prisoners and confronts them after Paltraki has led most of them out of the building. Graham considers shooting Tim Shaw, but decides against it and, with Ryan's help, puts the villain in one of his own stasis pods. Ryan and Graham tell Tim he's not worth killing and leave Grace's name as the last thing he hears before the pod is activated.
On Paltraki's ship, the Doctor offers to spare him a trip by taking some of the people home, but he refuses, as he's fine with doing it himself. Delph and Andinio decide that they ought to travel the universe so they understand it better. The Doctor heads off with her "fam" to see more worlds.
- Agony of the Feet: Graham shoots Tim Shaw in the foot to get him to shut up.
- And I Must Scream: Tim Shaw ends up sealed in one of his own stasis pods. Don't feel too sorry for him, though; he really had it coming.
- Big Damn Heroes: Paltraki, saving Ryan, Graham, and his crew from the sniperbots.
- When talking about the capabilities of the TARDIS, the Doctor mentions that she once pulled the Earth halfway across the universe and turned a Slitheen into an egg.
- Indeed, Tim Shaw's Ux-assisted planetary theft via stealing them from time and space is quite reminiscent of what Davros and the Daleks were doing in "Journey's End". The fact that the planets were shrunk down also echoes the Fourth Doctor serial "The Pirate Planet".
- The Doctor connects cables inside the TARDIS to machinery outside. It works out much better than it did in "Utopia".
- Chekhov's Gun: The Doctor has heard that being on Ranskoor Av Kolos has a negative effect on people's sanity and gives her companions psychic blockers as a precaution. She later uses them to free one of the Ux from being imprisoned in Tim Shaw's psychic-amplifying superweapon.
- Continuity Nod:
- The Doctor yet again mentions that Stenza technology doesn't make very much sense.
- The communication devices the team use are explicitly compared to the commdots seen in "The Tsuranga Conundrum".
- Paltraki and his crew come from the Nine Planets, presumably the same as the Nine Systems that Tim Shaw mentioned the Stenza as ruling in "The Woman Who Fell to Earth", hinting that maybe the Stenza's empire has fallen in the millennia since.
- While summoning the TARDIS into the Ux's shrine, the Doctor refers to it as her Ghost Monument.
- Ryan finally returns Graham's fistbump.
- Cruel Mercy: Tim Shaw lives through the episode, but is trapped in one of his own stasis pods and, according to the Ux, the shrine is sealed so that no one will ever be able to free him.
- Dark Lord on Life Support: Tim Shaw, as a result of the DNA bombs that the Doctor tricked him into absorbing the last time he appeared and never making it back to his homeworld thanks to the Doctor unintentionally corrupting his recall teleport.
- Deadly Dodging: When Graham and Ryan find themselves pinned between two groups of sniperbots, they lie on the floor and let the robots shoot each other.
- Distant Prologue: The first scene takes place 3,407 years before the rest of the plot.
- Distress Call: The Doctor and company come to Ranskoor Av Kolos because they pick up nine separate distress calls from the planet.
- Facial Markings: The Ux have spiral-shaped raised lines similar to scarring on their cheeks.
- Fate Worse than Death: Tim Shaw ends up trapped in a Stenza trophy pod while still conscious. What's more, the last thing he ever hears is "Grace" from both Graham and Ryan for him to dwell on forever.
- Finger in the Mail: Threatened by Tim Shaw; if Paltraki doesn't cooperate, his crew will be teleported onto his ship, piece by piece.
- A God Am I: Tim Shaw has been suffering delusions of godhood as a result of the Ux worshipping him. Ironically, it's the humble, pious Ux who actually have godlike powers which Tim is all too happy to exploit.
- Hate Sink: Tim Shaw is even worse here than in "The Woman Who Fell to Earth". When he's not endangering planets and forcing the innocent Ux to do his dirty work, he's being an insufferable Smug Snake who enjoys taunting the heroes; for example, he tells the Doctor that she's responsible for his atrocities because she let him live last time.
- Hive Mind: According to Tim Shaw, the Stenza have one, which allows him to recreate their technology.
- Hostage for MacGuffin: Tim Shaw murders one of Paltraki's crew, then tells him that he has until lightfall to deliver the MacGuffin if he wants the other two alive and in one piece.
- Human Aliens: Paltraki and his crew, as he doesn't recognize the name of Earth.
- I Don't Like the Sound of That Place: Ranskoor Av Kolos translates to "Disintegrator of Souls".
- If You Kill Him, You Will Be Just Like Him: The Doctor uses this argument when she learns that Graham is planning to kill Tim Shaw. The Doctor doesn't like murders, no matter the reason. She much prefers Cruel Mercy.
- Imperial Stormtrooper Marksmanship Academy: Once again, the incredibly inaccurately named "Sniper Bots" are incapable of hitting anything at all. Graham and Ryan outsmart them simply by crouching down. Technically, the Sniper Bots did hit something each other!
- My God, What Have I Done?: The Doctor straps a pair of grenades to the MacGuffin, intending it as a precaution if Tim Shaw tries anything. When she finds out that the MacGuffin is a planet, shrunk to a fraction of its size and kept in crystal as one of Tim Shaw's trophies, she's clearly horrified by how close she came to committing planetary genocide.
- No MacGuffin, No Winner: The Doctor straps a couple of grenades to the crystal so she can use it as a bargaining chip.
- Noodle Incident: The Doctor isn't sure, but she might have half-invented wellies.note
- Not Quite Dead: It turns out that Tim Shaw survived his previous encounter with the Doctor, though only barely. While the rest of the team are surprised that he's alive at all, the Doctor is only confused as to how he ended up here, at this point in time, as opposed to his home planet, confirming that she never intended to let him die when giving him back his recall teleport.
- Not Worth Killing: Graham and Ryan both decide against killing Tim Shaw, with Graham using the exact phrase. Instead, they sentence him life (in prison).
- Pacifism Backfire: As it turns out, the Doctor thought she sent Tim Shaw back to his home planet; her companions assumed she killed him. Instead he landed on a planet where he was able to manipulate two very powerful psychic aliens to do his bidding.
- Powered by a Forsaken Child: Tim Shaw uses the Ux's abilities to power a weapon strong enough to destroy planets. It especially applies to Delph, who was trussed up in the circuitry powering the weapon due to his doubts about Tim being the Ux's Creator.
- Phrase Catcher: While the official companions never say it, the Ux maintain the tradition. When Delph step into the TARDIS he immediately proclaims that it is "dimensionally transcendental", which is just a fancy way of saying "bigger on the inside than the outside".
- Reality-Breaking Paradox: The presence of four planets, shrunk by Tim Shaw's technology into trophies and kept in close proximity to each other, threatens the stability of the universe.
- Reality Warper: The Ux have the power to affect the shape of the universe through thought. Tim Shaw melds Stenza technology with this ability to enable them to place entire planets in a stasis crystal that can be carried by a person.
- Really 700 Years Old: The Ux are stated to have lifespans that last for millennia, and the same pair have been alive for the 3407 years between Tim Shaw teleporting onto their world and the Doctor receiving multiple Distress Calls at the start of the episode.
- A major motivator for Tim Shaw, who plans a co-ordinated weapon attack against every planet that has ever wronged or defied the Stenza. When the Doctor shows up, he targets Earth just to get back at her.
- Also for Graham, who is initially intending to kill Tim Shaw for his part in Grace's death earlier in the season.
- Rubber-Forehead Aliens: The Ux are indistinguishable from humans apart from markings on their cheeks.
- Shout-Out: Graham quips "Yippie Ki Yay, sniperbots!" when he detonates a bomb.
- Single Specimen Species: The Ux are a downplayed version of this trope. According to the Doctor, there are only ever two of them at a time.
- Supernatural Gold Eyes: The Ux's eyes glow golden when they're using their powers.
- A Taste of Their Own Medicine: Tim Shaw ends up frozen in one of the stasis pods he used to hold his trophies. Graham uses the trope name as he leaves him in the pod.
- Technical Pacifist:
- The Doctor's policy on weapons for this episode is that she's OK with them as long as they're used to destroy things that can be rebuilt.
- Even though he decides against killing Tim Shaw outright, Graham still puts a shot into his foot for good measure.
- Thou Shalt Not Kill: Graham intends to kill Tim Shaw. Both the Doctor and Ryan try to talk him out of it. Eventually, Graham decides to be "the better man" and lets Tim live, although Tim probably wishes that he was dead considering what happens to him.
- Tranquil Fury: Graham is eerily calm about seeing the Stenza again, and about wanting to kill Tim Shaw.
- The X of Y: "The Battle of Ranskoor Av Kolos".