A brutal mercenary police force, the rhinoceros-like Judoon, who have no jurisdiction on Earth, transport the hospital to the Moon using a "H2O Scoop" in order to apprehend and execute Florence Finnegan, a shapeshifting, bloodsucking Plasmavore with the ability to appear as whatever species whose blood she consumes. As soon as the hospital is in position, the cylindrical Judoon ships land and the troopers march into the building (due to their bodily shape and heavy footwear, their motion is referred to throughout the script as "yomping"). The Judoon begin scanning everyone in the hospital, cataloguing the humans whilst attempting to find the non-human criminal. Finnegan attempts to evade capture by sucking the blood of humans. Her first victim is Mr Stoker, the head doctor, and consequently she registers as a human when scanned by Judoons. The Doctor is identified as "non-human" and thus is thought to be the fugitive by the Judoon, forcing him and Martha, with whom he teamed up shortly after arriving on the Moon, to flee.
The Doctor and Martha discover Stoker's body drained of blood which confirms the Doctor's suspicion that a Plasmavore is in the hospital, this in turn explaining the Doctor's presence there to the audience. The Doctor kisses Martha, performing a "genetic transfer" which provides her with traces of contact with a non-human. He leaves her behind to be fully scanned by Judoon, providing him with more time as he goes in pursuit of the Plasmavore. Thinking the Doctor is human, Finnegan sucks his blood causing him to collapse unconscious, presumed dead. Judoons arrive and confirm the non-human is deceased. Martha enters, realises that Finnegan has absorbed non-human blood, grabs a Judoon's scanner and identifies Finnegan as non-human. Confirming Finnegan is a Plasmavore, the Judoon execute her for the murder of the child princess of Padrivole Regency Nine.
Before expiring, Finnegan modified an MRI scanner to make it destroy all life on the Moon and on the half of Earth facing the Moon except herself, thus allowing her to use the Judoon ships to escape. Their job done, the Judoon evacuate. Martha uses CPR on the Doctor's two hearts and manages to revive him, and he turns off the scanner. Oxygen levels are low and the occupants of the hospital are near death but, as they depart, the Judoon return the hospital safely to Earth.
Martha attends her brother's birthday party, but it breaks down into a family row. She notices the Doctor outside and follows him. He goes to the TARDIS and invites Martha to go on a trip with him as a reward for her help. Martha hesitates until the Doctor says he can travel in time as well, and proves it by travelling back to the previous morning and taking off his tie in front of Martha (later explaining, "Crossing into established events is strictly forbidden — except for cheap tricks"). After this demonstration, Martha steps into the TARDIS (noting that it's bigger on the inside). The Doctor refers to Rose Tyler and emphasises that Martha is not replacing her but is only going to get one trip as he prefers to travel alone.
