Present-day Cardiff is the site of a new public works project, Blaidd Drwg, a massive nuclear reactor to be built on the site of the present Cardiff Castle. However, when an investigator gets too close to the truth about it, he has a run in with the city’s new mayor… Margaret Blaine (Annette Badland), the last Slitheen survivor on Earth after their battle with the Doctor (“World War Three”).

The TARDIS arrives in Cardiff as Mickey (Noel Clarke) is there to meet them; he’s brought Rose’s passport, and now the four – Mickey, Rose (Billie Piper), Jack (John Barrowman) and the Doctor (Christopher Eccleston) – are able to spend a little time on vacation. It doesn’t take long, however, for the truth to out; the Doctor spots Margaret’s face in a newspaper.

Margaret avoids killing a local reporter, Cathy Salt (Mali Harries), after feeling sorry for the girl; it seems Margaret is developing emotions she’s not used to. However, when the Doctor and his friends come to call, she makes a quick getaway… or so she thinks, as the Doctor is able to outwit her. They take her aboard the TARDIS, where the Doctor says he will return her to Raxacoricofallapatorius. But she’s under death sentence, and anyway, the TARDIS is refueling from temporal energy excised by the Gelth rift (“The Unquiet Dead”), so they must stay overnight. Margaret says she must eat, so the Doctor takes his prisoner to dinner.

As Jack attempts to connect a piece of technology found in Margaret’s office – a ‘surfboard’ type of object that creates an energy bubble that allows a rider to sail shock waves – Mickey and Rose spend some time together. Mickey tells her that he’s dating again, but he’s come for one last chance to patch things up with Rose. Elsewhere, Margaret bids the Doctor for her freedom, and preys on his feelings about death.

Suddenly, the Gelth rift starts to open; Margaret takes advantage of this, attempting to seize control of the TARDIS. However, the heart of the TARDIS opens to her and regresses her back into an egg. Mickey decides to let Rose go, grudgingly, and the Doctor, Jack and Rose set off to deliver Margaret in her egg state back to her homeworld, where she will hopefully live a second life different from her first.