The Doctor, along with hundreds of staff and patients, is transported to the Moon when an entire London hospital is inexplicably relocated. The hospital is under a protective barrier and oxygen is limited. With the arrival of the Judoon, a sort of intergalactic police force, it becomes obvious they are looking for an extra-terrestrial being. In the chaos of the hospital, the Doctor meets Martha Jones who is fascinated by what she sees and what is happening to them. When one of the physicians, Mr. Stoker, is found dead it appears there is in fact a blood sucking being, a Plasmavore, among them which they must identify if the Earthlings are going to survive. For the Doctor, who is still pining for Rose, Martha might prove to be a most capable companion.
—garykmcd