The TARDIS falls into a massive gravitational field while undergoing repairs, and the Doctor and Romana encounter and repair a damaged spaceship. Romana is imprisoned after discovering that the ship is carrying a group of young slaves on board. The Doctor follows the ship to the planet Skonnos, where the youngsters are to be sacrificed to the Nimon, a bull-headed alien that has promised wealth and power to the Skonnons. Romana is transported to a dying planet, Crinoth, where she discovers that the Nimon on Skonnos is preparing for the migration of the entire race of the Nimon from Crinoth to Skonnos, having exhausted Crinoth's resources. Can she get back and warn the Doctor?