Martha Jones is heading to the hospital for her stint as an intern and on the phone dealing with her brother, sister, mother, and her father and his new girlfriend. A mysterious strange (the Doctor) comes up to her and gives her a tie. As she goes into the hospital, she bumps into a biker wearing his helmet who rudely brushes past her. As she goes to her locker, she notices a buildup of static electricity.
Martha goes on her tour with Mr. Stoker, the head doctor, who is diagnosing Miss Finnegan. They then meet with a Mr. Smith (also the Doctor), a patient, and he denies having met Martha earlier. Martha determines he has two hearts and they notice more static electricity due to the coming storm. Martha calls her sister Tish, who is outside and reports that a storm has formed over the hospital, then the rain goes up. The hospital shakes as there’s a flash of light and when Martha and the others recover, they determine that they’ve been teleported to the Moon.
Everyone is able to breathe but people start to panic. Martha realizes that they’re in a force field, impressing the Doctor. Martha concludes extraterrestrials are involved and realizes that there is nothing to renew the air and they’ll eventually suffocate. Three large alien ships then land nearby and armored aliens march on the hospital, and the Doctor identifies them as the Jadoon. Mr. Stoker is watching them when Miss Finnegan approaches him with two of the bikers, and takes out a straw to suck his blood while the bikers hold him.
The Jadoon enter the hospital, master the English language, and catalog them as human. The Doctor, in hiding, explains to Martha that the Jadoon brought the hospital to the Moon as its neutral ground. The Jadoon move through the hospital, killing one man who interferes with them. The Doctor, tapping into the computers to find unusual patient records, explains to Martha that he was passing by and noticed the energy buildup, and checked himself in to investigate. Unfortunately, the Jadoon have wiped the records. Martha goes to find Stoker and Finnegan’s bikers go after them. The Doctor and Martha run through the hospital to the X-ray lab. The Doctor rigs the projector and bathes himself and one of the bikers with radiation. It destroys the biker (a “slab”, a synthetic slave life form) and the Doctor expels the radiation into his left shoe and gets rid of it. Martha explains that Finnegan is the alien and the Doctor realizes she was assimilating human blood so she can pass as human. The Jadoon find and scan her and confirm that she’s human.
The remaining slab continues its search and the Jadoon find the Doctor and confirm he’s non-human. The Doctor and Martha run away and head for a floor the Jadoon have already searched. The doctors and patients are starting to collapse from suffocation, and the Doctor and Martha find Stoker’s body. The Doctor then heads for the MRI scanner room and kisses Martha, then tells her to stall the Jadoon. He finds Finnegan in the MRI chamber as he figured and pretends to be human, and Finnegan has her last slab hold him.
The Jadoon find Martha and detect traces of the Doctor on her lips. Meanwhile, Finnegan explains she’s rigging the MRI to kill every living thing within range except for herself, safe in the chamber. The electronic wave will also kill everyone on the facing side of the Earth. She then prepares to drink the Doctor’s blood.
The Jadoon release Martha and continue their search, entering the MRI chamber as Finnegan finishes feeding from the Doctor. The Jadoon scan him and confirm that he’s dead. Martha points out that Finnegan isn’t human and scans her: the Doctor’s blood registers her as non-human. Finnegan sends the slab to distract them while she activates the MRI chamber. The Jadoon kill her then leave to avoid the magnetic wave, their mission complete. Martha resuscitates the Doctor using mouth-to-mouth before collapsing due to lack of air. The Doctor shuts down the MRI chamber, and the Jadoon reverse the teleport and the hospital reappears back in London. The EMTs deal with the patients while the Doctor slips away in the TARDIS.
The Jones family party doesn’t go well that night and everyone storms off. The Doctor is watching from an alleyway and Martha follows her to where the TARDIS is parked. He offers her a trip in return for saving his life. When she doesn’t believe him, he briefly dematerializes the TARDIS then returns with his tie, proving that he went back to see her that morning. He takes her inside and she’s duly impressed. The Doctor briefly mentions Rose as a former passenger, then prepares to take her on her one trip… which starts as a bumpy ride.