The locator leads the Doctor and Romana to the third moon of Delta Magna.

Long synopsis: The locator leads the Doctor and Romana to the third moon of Delta Magna. The moon is a swamp covered world on which a small crew of humans maintain a refinery and are in constant feud with the simpler humanoid creatures known as swampies who also live there. Their God, a giant octopus like creature called Kroll makes an appearance and threatens the refinery.