The inhabitants of the planet Skonnos have been promised by an alien Nimon that he will restore their empire to greatness if they in return provide young sacrifices and radioactive hymetusite crystals, both of which they are obtaining from the nearby planet Aneth.

With the TARDIS immobilised for repairs, the Doctor and Romana encounter the Skonnan spaceship transporting the latest sacrificial consignment from Aneth. Romana is captured and taken to Skonnos on board the ship, while the Doctor follows in the now semi-repaired TARDIS. Once there, they are consigned by the Skonnans' leader, Soldeed, into the Nimon's labyrinthine power complex.

The two Time Lords discover that the Nimons are a race of parasites moving from planet to planet like a plague of locusts. The bull-headed creatures send a lone representative to an unsuspecting world, offering assistance in order to gain the trust of its inhabitants, then arrive in force to drain it of its resources.

The power complex, fuelled by hymetusite, uses a black hole to create a space tunnel through which the Nimons cross from one planet to the next.

The Doctor manages to destroy the complex on Skonnos, thereby trapping the Nimons on their last home, the dying planet Crinoth.