Mickey Smith arrives in Cardiff, where the TARDIS has materialised in front of the water sculpture in Cardiff Bay’s Oval Basin. Rose has asked him to fetch her passport; she won’t need it in places such as Platform One, Justicia or the Glass Pyramid of San Kaloon, but now she’s prepared to travel to other countries on Earth. Mickey notes to his disappointment that Rose isn’t back for good; the TARDIS has just touched down in Cardiff to refuel from the energies emanating from the scar created when a dimensional rift was sealed up in 1869. By now, the Doctor, Rose and Captain Jack are a well-rehearsed team, and it’s clear that Mickey isn’t really a part of it. Nevertheless, he accompanies them back outside, where the Doctor explains to the curious Captain Jack that a police box was used in 1950s London not only as a phone booth, but also to restrain offenders until reinforcements arrived. He assures the worried Mickey that nobody will notice the TARDIS here, even though a police box isn’t exactly a perfect disguise for 21st-century Cardiff; he’s just grown to like its shape and never bothered to fix the broken chameleon circuit. It will take 24 hours for the TARDIS to refuel, and the Doctor and his friends -- and Mickey -- set off for a day on the town.

Elsewhere in the city, Margaret Blaine is holding a press conference to announce that the Blaidd Dwrg project is ready to begin, but is irritated when one of the photographers snaps a picture of her. Soon Cardiff Castle will be demolished and replaced by a nuclear power station in the heart of Cardiff, providing jobs for all. Margaret assures the press that the station is perfectly safe, and raises a toast to Cardiff’s bright, glowing future. She has already asked the press to concentrate on the project rather than on her, but Cathy Salt of the Cardiff Gazette nevertheless asks for her impressions about the so-called “curse” that has struck down so many people associated with the project. A series of freak accidents have taken out the entire European safety inspection team, the Cardiff Heritage Society, the architect of the project, and now Mr Cleaver, the government’s nuclear advisor. Before Cleaver died, however, he posted some of his findings online -- and Cathy has located them and found that he was concerned that the Blaidd Dwrg project could go into meltdown and kill millions.

Margaret agrees to have a word with Cathy, but first they pop into the ladies’ loo, as Margaret’s stomach is rumbling. The Slitheen have always had this problem, caused by the gas compression field they use to squeeze into their human skin-suit disguises. Margaret enters a stall and strips out of her skin-suit, telling the concerned Cathy that the flashing, flickering lights caused by the compression field are due to a fault in the room’s wiring. Cathy explains that Cleaver feared the nuclear complex could go up in a disaster worse than Chernobyl, and tells Margaret that she has no choice but to go to print with her findings. But just as the Slitheen is about to emerge from the stall and strike Cathy down, Cathy mentions in passing that her fiancé, Jeffrey, thinks she’s mad to be causing a fuss. Margaret stays her hand, especially when Cathy also announces that she’s three months pregnant. Noting the sad tone in Margaret’s voice, Cathy inquires after her family, and Margaret admits that she used to have quite a large family -- but they’re all gone now. Claiming that she’s going to be some time in the loo, Margaret tells Cathy to set up an appointment with her for later, and as Cathy leaves, the despondent Margaret remains in the stall, suddenly feeling weary and alone.

The Doctor and his friends are dining out at a restaurant on the pier, and even Mickey is enjoying himself as Captain Jack shares wild stories from his past. But the happy day out is interrupted when the Doctor spots a nearby newspaper with a front-page photograph of Margaret Blaine, which was snapped at the earlier press conference. He and his allies head straight for City Hall, where Rose, Jack, and the increasingly nervous Mickey split up to cover the exits while the Doctor confronts the Mayor in person. Outside her office, he asks her secretary, Idris, to tell the Mayor that the Doctor has arrived -- and once Margaret hears this, she flees out the window onto the balcony, removing items of jewellery from her person and snapping them together into a personal teleport. Idris tries to hold the Doctor back, but the Doctor pushes past him, calling his allies on his cell phone. Rose and Jack rush into position, but Mickey hesitates for a moment and then runs straight into a cleaning woman’s cart, tripping himself up. Margaret thus gets past him and activates her teleport, vanishing into thin air. However, the Doctor activates his sonic screwdriver, bringing her straight back again, and after another few futile attempts to activate her teleport, Margaret surrenders.

The Doctor marches Margaret back to the Lord Mayor’s office, where he demands to know why the last Slitheen on the planet has arranged to build a nuclear power station in the heart of Cardiff, right atop a dimensional rift. He takes one look at the model and realises that the station is designed to explode the moment it reaches capacity. Rose wonders why nobody has noticed, and Margaret scoffs, pointing out that nobody in London cares what happens to Cardiff. The Doctor removes the central building in the model to reveal that it’s been built atop a tribophysical waveform macrokinetic extrapolator, a sort of pan-dimensional surfboard. Jack is awed and excited by the technology, which he assumes, probably correctly, that Margaret has stolen. Margaret needs only to programme the correct co-ordinates into the board; when the station explodes, opening the rift and destroying the Earth, the board will generate an energy bubble around her, and she will be able to surf the energy wave away from this backwater planet back out into the heart of civilisation. As she reveals her true plans, however, the Doctor’s attention is caught by the project’s name. Margaret claims to have picked it at random because she liked the way it sounded, but the Doctor reveals that Blaidd Dwrg is Welsh for “bad wolf,” a phrase that seems to have been following him and Rose around on their travels...

After a moment’s thought, the Doctor dismisses this as a coincidence, and prepares to put an end to Margaret’s scheming by taking her back to Raxacoricofallapatorius. But Margaret reveals that her people have the death penalty, and her family has been tried and found guilty of their crimes in absentia. If the Doctor takes her back home, he’s taking her to her death. However, the Doctor tells her that this isn’t his problem. He and his allies take her back to the TARDIS, where Margaret seems awed by the godlike technology on display; however, the Doctor doesn’t succumb to flattery. Jack wires the extrapolator into the console; its power is incompatible with the TARDIS systems, but with some adjustment, it will boost the TARDIS engines and they’ll be ready to go by morning. However, that means that they’ll have to spend the night guarding Margaret. Rose jokes that the police box really is acting like a police box, but Margaret deflates her by pointing out that they’re not acting as her captors, but her executioners. Mickey is quick to say she deserves it, but when she challenges them to look her in the eye, none of them can do so -- not even the Doctor.

Mickey eventually leaves, unable to cope with the tense atmosphere inside and finding it easier to convince himself that Margaret deserves her fate when he doesn’t have to look at her. Rose follows him and admits that she didn’t really need her passport, and Mickey, pleased, suggests that they go get a drink and a pizza... and maybe, if Rose is willing, they could book a hotel room for the night. Inside the TARDIS, as the Doctor glumly watches Mickey and Rose walk off hand in hand, Margaret challenges him again, having realised that he always leaves before he has to face the consequences of his actions. The Doctor insists that he didn’t butcher her family, as she puts it, but when Jack asks her why she didn’t save them as well, she explains that her teleport can only transport one at a time; without any time to set co-ordinates, she ended up in a skip on the Isle of Dogs. The Doctor and Jack laugh at this despite themselves, and Margaret takes the opportunity to ask for a last request; she claims to have grown attached to her human life, and before leaving, she wants a final meal at her favourite Cardiff restaurant. The Doctor is reluctant to let her out in public again, but Jack provides them with limiting bracelets; if Margaret gets more than 10 feet away from the Doctor, she’ll be zapped with 10,000 volts of electricity.

The Doctor thus agrees to escort Margaret to the nearby bistro 10. There, she introduces herself properly to him as Blon Fel Fotch Pasameer-Day Slitheen, reasoning that he should know the real name of the person he’s condemned to die. She tries to kill him by slipping poison into his wine; when this fails, she fires a poison dart out of her finger at him, but he plucks it out of mid-air and sprays mouthwash at her when she tries to exhale the excess poison into his face. He then orders steak and chips. Margaret tries pleading, telling the Doctor that she will be executed by being dissolved in a vat of acetic acid while still alive. There are other members of the Slitheen family making a life for themselves off Raxacoricofallapatorius, but the Doctor refuses to take her to them instead, convinced that she’ll return to her old ways. She may beg for her life, but the Doctor reminds her that she’s doing so while dressed in the skin of the real Margaret Blaine, one of her victims. Margaret insists that she can change, and tells him that she allowed Cathy Salt to live even though the journalist was a threat to her plans. However, the Doctor knows that this was just a whim; occasionally Margaret allows one of her victims to live, just so she can live with herself while going on to commit greater atrocities. Margaret, however, realises that the Doctor’s insight comes from personal experience. He’s just as much a killer as she is, which is why he keeps moving on, so he won’t have to stay and live with the consequences of what he’s done. So if she can allow someone to live on a whim, then why can’t he?

Out on the docks, Rose is telling Mickey about her visit to the planet Woman Wept, named after a major landmass in the shape of a crying woman, where she and the Doctor walked on a frozen sea under hundred-foot-high waves of ice. Mickey has nothing to tell her that compares to this, and, out of the blue, he reveals that he’s started dating Tricia Delaney. There is an uncomfortable silence, and when Mickey asks Rose if she still wants to get a hotel room, she lashes out at him, more upset than she’d expected to be by the news. However, Mickey angrily reminds Rose that they had a good relationship, at least until she left him without a second thought to travel with the Doctor. She made him feel like he meant nothing to her, and yet when she called him here, he still came running. He begs Rose to give him a promise that she’ll return to him when she’s finished travelling, but before Rose can respond, she hears something like a sound of thunder -- and the streetlights on the pier begin to explode. Pedestrians flee in panic as the street shakes and the buildings begin to crumble, and Rose rushes off to investigate without pausing to think -- and without hearing the bitter Mickey shouting after her, now knowing that the Doctor will always mean more to her than he does.

At bistro 10, as Margaret is now trying to convince the Doctor that her family made her this way; her father forced her to kill when she was 13, under threat of being fed to the venom grubs. But their conversation is also interrupted by the sound of thunder, and then by the shattering of the bistro window. The Doctor rushes out to investigate, and since Margaret can’t keep up with him, he removes her limiting bracelet when she assures him that she’s not going to let him out of her sight until they know what’s happening. It soon becomes clear when they get back to the TARDIS to see energy blasting up out of the ship and into a growing hole in the night sky. The dimensional rift is opening again. Margaret and the Doctor rush up to the TARDIS as the Oval Basin cracks beneath their feet, and enter to find Jack struggling to get the waveform extrapolator back under control. He’s disconnected it, but it’s still feeding off the TARDIS systems somehow -- and if they can’t shut it down, the rift will open and the entire planet will be torn apart.

Rose rushes into the TARDIS, right on cue for Margaret to rip off her human arm disguise and grab Rose by the throat. She now orders Jack to place the extrapolator at her feet or see Rose’s neck snap. Margaret always knew there was a possibility that she’d be tracked down here, but reasoned that whoever did so would probably be advanced enough to recognise the waveform extrapolator for what it was and take it from her; thus, she programmed it to lock onto any nearby alien energy source and use it to open the rift. Margaret steps onto the extrapolator, intending to ride the destruction of Earth to freedom -- but the TARDIS console opens up to reveal the blinding light inside. The Doctor warns Jack and Rose to look away, and informs Margaret that the ship is alive and knows that Margaret is trying to destroy it. Margaret lets Rose drop to the floor, captivated by what the Doctor claims is the heart and soul of the TARDIS. For a moment she smiles at the Doctor, a genuine smile of thanks, and then the light flares and her empty skin-suit drops to the ground.

The Doctor leaps back into action and closes up the console, and he, Jack and Rose work together to shut down the TARDIS engines. The TARDIS stops pouring energy into the rift, which snaps shut again. Jack assumes that Margaret has been incinerated, but the Doctor admits that the TARDIS is stronger than even he can imagine -- and, as he once told Rose, it’s telepathic as well. It can translate alien languages at a distance; perhaps the power at its source can also translate thoughts and desires. The Doctor opens up the skin-suit to reveal a Raxacoricofallapatorian egg inside; Blon has been regressed to childhood and beyond, and if they take her back home to the nurseries, she can be raised by another family and have a second chance at life.

Rose suddenly remembers that she abandoned Mickey on the docks, and rushes out to look for him. Mickey is lurking in the shadows, watching as the emergency services move in to deal with the aftermath of the unexpected quake -- and when he sees Rose searching for him, he turns his back and walks away without a word. Rose eventually returns to the TARDIS alone, where the Doctor tells her that the TARDIS is now full of energy from the open rift and is ready to go. He offers to stay while she looks for Mickey, but she tells him that there’s no point. The Doctor shrugs, and he and Jack set the TARDIS on course for Raxacoricofallapatorius while Rose stares sadly at the egg, wishing for a second chance of her own.