A London hospital along with its staff and patients is transported to the Moon. Three spaceships land nearby, and the hospital is invaded by the Judoon, an intergalactic police force for hire who are searching for a Plasmavore, an alien with the ability to appear as the species whose blood it consumes. The Judoon begin scanning everyone in the hospital, cataloguing the humans whilst attempting to find the non-human criminal. The Tenth Doctor, posing as a human patient to investigate the hospital, talks to medical student Martha Jones, revealing that he is an alien as well. The two go down to the lobby to find out what the Judoon are doing. An older lady named Florence Finnegan reveals that she is the Plasmavore. She drains Mr Stoker, the head of the hospital, and consequently registers as human when the Judoon scan her. The Doctor knows if he is scanned he will register as non-human, so he and Martha quietly avoid the Judoon, who will also likely execute all people within the hospital on grounds of harbouring a fugitive if they discover a non-human in the building. As the oxygen level in the hospital drops, people begin to collapse.
The Doctor finds Miss Finnegan in an MRI room, who is modifying the scanner to make it destroy all life on the Moon and the half of the Earth currently facing it. The Doctor pretends to be a confused human and Miss Finnegan drinks his blood until he collapses. Martha enters and grabs a Judoon scanner and exposes Finnegan as non-human. Confirming Finnegan is the fugitive they seek, the Judoon execute her for the murder of an alien princess and return for their ships. Martha uses CPR on the Doctor’s two hearts to revive him. The Doctor disables the modified MRI and the Judoon finally shift the hospital back to Earth seconds before everyone perishes from oxygen deprivation.
That evening, after a birthday party for Martha’s brother Leo ends in a fight, the Doctor invites Martha to go on a trip with him Martha hesitates until the Doctor says he can travel in time as well. Martha steps into the TARDIS, and the Doctor tells her that she is only going on one trip with him.