The TARDIS returns Rose Tyler home to her own time, but Rose is startled to discover that, instead of the twelve hours the Doctor thought she was gone, it's been twelve months, and Rose was listed as a missing person. Jackie Tyler (Camille Coduri) blames the Doctor for her daughter's wanderlust, but Rose is unable to tell her mum what's happened to her. Later, as Rose muses that she's the only person on Earth who knows what she knows, an alien spacecraft suddenly flies overhead, crashes through Big Ben and lands in the Thames... mankind's first public encounter with aliens. Or is it? The Doctor and Rose can't get close to the action, so they take Rose's suggestion and watch it on BBC Television, where they keep up to date with news reports that one body has been taken from the wreckage by the military to the local Albion Hospital.

Inside the hospital, military commander General Asquith (Rupert Vansittart) is shown the alien body by pathologist Dr. Sato (Naoko Mori). At 10 Downing Street, Joseph Green (David Verrey), a low-level MP, has been appointed as the acting Prime Minister, because the current Prime Minister has disappeared. Another MP, Harriet Jones (Penelope Wilton) is waiting for a meeting with the PM, but is told by junior secretary Indra Ganesh (Navin Chowdhry) that more important things are happening now. Indra escorts Green to a meeting, where Green is to meet with Margaret Blaine (Annette Badland) of MI5 and transport liaison Oliver Charles (Eric Potts)... but inside, Green, Blaine and Charles seem to known each other and laugh, especially since they now possess the emergency protocols dealing with alien invasions.

While Rose deals with her family, the Doctor leaves temporarily, promising he won't abandon her and giving her a TARDIS key. The Doctor heads back to the TARDIS while the revelry over the alien landing occurs, but he's spied by Mickey (Noel Clarke), Rose's boyfriend, and follows him. But the TARDIS departs before Mickey can get to the Doctor. Mickey then comes to Rose's flat, where she learns that he was questioned by the police over the past year and made by Jackie to be a suspicious character, perhaps even her murderer.

At Albion Hospital, Dr. Sato is attacked by the strange creature who is currently in one of the morgue shelves. The TARDIS arrives in the hospital, and the Doctor finds Sato on the floor. Confronted by military soldiers, the Doctor begins giving them orders to secure the facility, and then discovers the creature that attacked Sato... a giant pig, which runs down the hallway before one of the guards shoots it down in fear. The Doctor and Sato discover that the pig has been technologically augmented and placed in the ship; it's not an alien, but in fact an ordinary pig. Could this alien landing be some sort of a massive joke?

Harriet Jones enters the meeting room at 10 Downing and sees the emergency protocols about the alien landing. She hides in a nearby closet, however, when Green, Blaine and Charles return to the room, having some severe gas control issues as they are confronted by General Asquith. In fact, the gas exchange is due to the fact that the three are not who they appear... as they reveal that their skins are simply external facades, and as Asquith discovers their alien identity -- the last discovery he will ever make. Harriet watches as one of them takes over Asquith's body, disposing of the Oliver shell.

The TARDIS returns to Rose's neighborhood, but Mickey and Jackie follow her inside. Jackie panics, and later calls an emergency number broadcast on the television for anyone who believes they've seen an alien. The words "Doctor" and "TARDIS" in her phone call sets off warning flags, as Indra Ganesh receives a "Code 9" indicating the presence of the Doctor on Earth. Meanwhile, the Doctor discovers that the alien spacecraft didn't follow a straight trajectory toward Earth, but in fact an elliptical course, having taken off from Earth, sent into space and then crash landing in London; he also discovers that the United Nations Intelligence Taskforce (UNIT) has been called in to advise. The Doctor, Rose and Mickey leave the TARDIS, where Mickey runs for cover and the Doctor and Rose are immediately taken into custody by the military.

At 10 Downing, the Doctor and Rose meet Harriet and Indra; while the latter takes the Doctor to the meeting with the UNIT representatives, Harriet confides in Rose about what she's seen, including the fake Oliver Charles skin the alien wore. Back at home, a police commissioner (Steve Speirs) interviews Jackie... but he also has some gas-related issues. Indra, Harriet and Rose discover the body of the Prime Minister in a closet in the meeting room and are suddenly discovered by Margaret Blaine. The alien inside Blaine reveals itself and chokes Indra, and the police commissioner attacks Jackie. Back at 10 Downing, Green uses a device which activates the electrical charges inside the Doctor and UNIT's ID badges, immobilizing them, while he and the alien inside Asquith reveal themselves... they are Slitheen, and have come to take over the planet Earth.

Extras

• Screen Caps

Thanks to Outpost Gallifrey for the story synopsis.