The planet Skonnos is dying, and its inhabitants have been promised by an alien Nimon that he can return their empire to greatness. However, this requires sacrifices and Hymetusite crystals, both of which the Skonnons obtain from the nearby planet Aneth. The Anethans have been supplying their young and their minerals for many years and the Nimon on Skonnos has become a part of their legends.

When the Doctor and his companions arrive - Romana first on the ship with the tribute and the Doctor and K-9 later by TARDIS - they are sent into the Nimon's labyrinthine Power Complex on Skonnos to meet the creature. It turns out to be a bull-headed alien. The Nimon are like a race of intergalactic locusts. They send a representative to an unsuspecting planet, offering help and assistance, and when they have gained the trust of the inhabitants, they arrive in force and drain the planet of all its resources before moving on to another world in the same manner.

Romana travels back along the Nimon's space-tunnel to their last world, the dying planet Crinoth, which they have all but exhausted. She obtains from Sezom, the last surviving inhabitant, a staff which can be used against the Nimon.

The Doctor manages to trap the Nimon on Crinoth by destroying the Complex on Skonnos.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}