An entire hospital is transported, along with all of its staff and all its patients, to the surface of the Moon. Three spaceships land nearby and the Judoon invade the hospital. The rhino-like aliens are an intergalactic police force for hire searching for a plasmavore, an alien capable of appearing to be the species whose blood it consumes. The Judoon begin to scan and catalogue everyone in the hospital.

The Doctor (David Tennant) poses as a human patient and talks to a medical student named Martha Jones (Freema Agyeman). He reveals that he is an alien as well and they go down to the lobby to talk to the Judoon. An older lady named Florence Finnegan (Anne Reid) reveals that she is the plasmovore, having consumed the blood of Mr Stoker, the head of the hospital (Roy Marsden). She thus registers as a human when the Judoon scan her. Knowing that he will register as alien if scanned, the Doctor tries to avoid the Judoon. Everyone in the hospital would likely be executed for harbouring a fugitive. People begin to collapse as the oxygen level drops.

The Doctor finds Miss Finnegan in an MRI room, fixing a scanner to destroy all life on the Moon and on the half of Earth facing it. The Doctor pretends to be human and she drinks his blood till he collapses. Martha uses a Judoon scanner and exposes Finnegan as non-human. The Judoon execute her for the murder of an alien princess and return to their ships. Martha Jones uses a CPR machine to restart the Doctor’s hearts. He then disables the modified MRI and the Judoon return the hospital to Earth just before the oxygen runs out.

That evening, after a fight breaks out at a birthday party for Martha’s brother Leo, the Doctor invites Martha to get away from it all. She steps into the TARDIS, properly amazed, and the Doctor tells her it is only for one trip.

Martha mentions her cousin Adeola, who appeared in the Army of Ghosts episode, played by Freema Agyeman herself—the role that led to her being chosen to play Martha Jones. David Tennant was given the phrase, “Judoon platoon upon the moon,” just because it is so hard to say with a Scottish accent. Ann Reid previously played Nurse Crane in the Curse of Fenric episode, a Tom Baker story about Haemovores, and in this story, she plays a Plasmovore, another version of the vampire. The hospital administrator was named Mr Stoker and on his door is printed B. Stoker because the design team thought he was supposed to be Bram Stoker.

A poster and some references on TV to Harold Saxon begin a season-long plot centred about this powerful man. Martha refers to the spaceship crashing into the Big Ben Tower in Aliens of London, and the various other alien invasions since. The Doctor uses his alias of John Smith—thus the title of the episode. The Pilot of the series Law and Order UK starred Freema Agyeman and was written by Doctor Who writer Chris Chibnall. It featured the Royal Hope Hospital. Several British hospitals appeared in this episode as parts of the Royal Hope. The episode was well received. The scary Judoon and the feisty Martha Jones were praised. She became a companion and a member of UNIT and Torchwood later on.