The Doctor (Christopher Eccleston) intends to return his companion Rose Tyler (Billie Piper) to London 12 hours after she left, but the TARDIS arrives 12 months after she left. Rose’s mother Jackie (Camille Coduri) is furious. She thought Rose had been abducted and murdered, and her boyfriend Mickey (Noel Clarke) is upset because he’s the one who was believed to have killed her. As Rose is complaining about not being allowed to tell what had really happened, a spaceship crashes through the Big Ben tower into the River Thames.

The Doctor suspects something is up and lands the TARDIS inside the hospital where the injured alien pilot is being treated. He discovers that the spaceship was launched from Earth and the so-called pilot is a physically modified pig. The government cannot locate the Prime Minister in the alien-landing confusion, and a member of Parliament, Joseph Green (David Berry), is appointed Acting PM. Green, however, is a member of the alien Slitheen family, who squeeze their bodies into suits made of dead humans. This, incidentally, causes extreme flatulence. Two more members of Parliament, Margaret Blaine (Annette Badland) and Oliver Charles (Eric Potts) are also Slitheen. but they are being watched suspiciously by Harriet Jones (Penelope Wilton), who knows what they are.

When the Doctor returns to Rose, her mother Jackie discovers the truth about him and the TARDIS and tells the authorities. The time-travellers are picked up and taken to 10 Downing Street, where the Doctor is put on a panel of experts on alien affairs, including members of UNIT, and Harriet Jones takes care of Rose. Together, they discover the corpse of the real Prime Minister, but they are caught by Margaret Blaine, who unzips her human suit and attacks them in her true alien form.

At Jackie’s flat, a police officer also unzips his human suit and attacks Jackie. As the Doctor tries to convince the panel that the spaceship crash was faked, he realizes that all the experts were gathered together at Downing Street in a trap. Acting MP Green assaults them with electricity. The episode ends, to be continued in the next episode, entitled World War Three.

The Slitheen were part CGI and part prosthetics. The story was inspired (There is a long list of these) by Quatermass and the Pit. They hired a Tony Blair impersonator to play the dead body of the PM, but he didn’t really look like Tony Blair, so they gave the camera only a glimpse of him. In this episode, UNIT stands for United Nations Intelligence Taskforce, but the name was changed to Unified Intelligence Taskforce in later episodes because the United Nations complained. Filming of the crashing spaceship raised the ire of anti-terrorist officers. The episode received mixed reviews. It was considered fart-joke silly, which it was, but delighted some reviewers and, I’m sure, most children. A UFO cake being presented on TV was made of jelly babies.

The scene in which the Doctor encounters the pig alien was Christopher Eccleston’s first scene filmed as the Doctor. When the Doctor is slapped by Rose’s Mum, Rose laughs, “You’re so gay!” and this raised some hackles, but writer Russell T. Davis, who is gay, said this is just the way young people talk. The words Bad Wolf are written on the side of the TARDIS by a child graffiti artist. Rose gets a key to the TARDIS. An actress named Naoko Mori plays Doctor Sato on the expert panel. She later played the same character on the Torchwood spinoff. She and Christopher Eccleston played Yoko and John Lennon in a movie called Lennon Naked. Annette Badland, who played a scary farting Slitheen, reported that she did all her own stunts. She said that children used to follow her in the shops, and when she reached up as if to unzip her face, they would run screaming.