The Doctor is in a hospital where he meets Martha Jones, a medical student. When the hospital and all it's occupants are taken to the moon, it's up to the Doctor, joined by Martha, to figure out what's going on. Then the Judoon show up looking for an alien. The Doctor is in trouble even though he's not the alien they're looking for. Can the Doctor work out a solution to the problem in time to save everyone before the air runs out?