Parts: 4
Writer: Anthony Read
Airdates: 22 December 1979 - 12 January 1980
Production Code: 5L

Doctor: fourth
Companions: the second Romana, K-9 mark two

Creature Feature: Nimon

Story: As a ship carrying the final sacrifices for the Nimon nears the planet Skonnos, it malfunctions and goes dead in space. A similar fate awaits the TARDIS, which is partially disassembled when it is caught in a gravity well. The TARDIS rams into the ship, so the Doctor, Romana and K-9 investigate.

They find that the ship holds several radioactive hymetusite crystals, along with about a dozen frightened teenagers from the planet Aneth. The Doctor helps the co-pilot of the ship to regain power, but before he can get the TARDIS onto the ship, the co-pilot resumes his course and strands the Doctor.

Arriving on Skonnos separately, the Doctor and Romana - accompanied by the Anethan sacrifices - find themselves trapped the Nimon complex by Soldeed, the lord of the land and servant of the Nimon. If they can navigate the ever-shifting maze of corridors and find the control area, they must then discover what plans the Nimon have for Skonnos, and how to stop them from sucking the planet dry.