The Doctor (Christopher Eccleston) lands the TARDIS in Cardiff, Wales, so he can use the energy of the Cardiff Rift to recharge the time machine. Mickey (Noel Clarke) joins the Doctor, Rose (Billie Piper), and Jack (John Barrowmam) for lunch. The Doctor notices a newspaper article showing that Margaret Blaine (Annette Badland) is Cardiff’s new Lord Mayor. He knows that she is really an alien named Blon Fel-Flotch Dassameer-Day Slitheen living in a human body. He thought she had been destroyed after trying to conquer the Earth six months earlier.

The four of them track her down and capture her. She has a scale model of a planned nuclear power plant, but the Doctor realizes it is designed to melt down and open the Cardiff Rift, destroying the Earth while powering her escape pod off the planet. He intends to take her back to her native Raxacoricofallapatronis, but there is a death sentence hanging over her head on that planet and she will be executed. He takes her to dinner, she tries to kill him a couple of times, ineptly, and asks him to take her to a different planet.

There is a tribophysical waveform macro-kinetic extrapolator included in the plans to use as her escape pod. Jack attaches it to the TARDIS to speed up the engine recharge. Cardiff is struck by an earthquake from the Rift. The extrapolator was a trap, directing the TARDIS energy into the Rift. Jack and the Doctor are unable to stop it, and Blaine takes Rose hostage to bargain for her freedom. But the Heart of the TARDIS opens, bathing Blaine in light. The Doctor and Jack quickly close the Rift.

Blaine’s human suit is empty except for an egg. The Doctor thinks the TARDIS gave a second chance at life to Blaine. They decide to bring the egg to Blaine’s home planet so she can be raised by a different family, ending the evil Slitheen line. Rose notices that her boyfriend Mickey has left her.

The storyline was about the Doctor’s reluctance to bring someone to their execution. It was also a travelogue about Cardiff, with scenes set in Cardiff Bay, the Millennium Centre, the Bistro 10 Restaurant, and the Water Tower at Roald Dahl Plass. In the Dahl Plass, named for the Welsh author, the Millennium Centre boasts these words in both Welsh and English: “Creating truth like glass from inspiration’s furnace” and “In these stones horizons sing.” Below the building is the Rift and the Torchwood Hub, which Jack had to lock down on this particular day so he would not accidentally run into himself in the Plass above.

Apparently, Christopher Eccleston and writer Russell T. Davies had a fight that came to blows and they had to be separated by the crew. The nuclear plant project is called Blaidd Drwg, Bad Wolf in Welsh. Rose mentions Gwyneth, a psychic in Cardiff that she and the Doctor met in 1869 (the Unquiet Dead episode), drawing her powers from the Rift. She was played by Eve Myles, who plays her descendant Gwen Cooper on Torchwood. The episode received mixed reviews. It was thought to be a minor story sandwiched between great ones. Because it was set in contemporary Cardiff and the fact that the Slitheen costume was already made provided a cheap episode to save money for the big series finale episodes Bad Wolf and Parting of the Ways.