The Doctor and Sarah respond to a distress call and the TARDIS materialises on Zeta Minor at the edge of the known Universe. A geological party from the planet Morestra has run into trouble, and only Professor Sorenson has survived.
A military party from Morestra has been sent to investigate the mysterious disappearances. The Doctor discovers that Sorenson has been attempting to take anti-matter from the planet, and an entity from the anti-matter universe has been retaliating.
The Doctor communicates with the entity and secures freedom for all on the condition that no anti-matter is taken from the planet. The Doctor convinces the military party to leave the anti-matter behind, and the ship manages to take off. Sorenson, however, has smuggled on board a consignment, and the ship is gradually dragged back to the planet..
Sorenson becomes infected by the anti-matter, and transforms into anti-man. The Doctor manages to get himself, Sorenson and the anti-matter back onto the planet. He throws Sorenson and the anti-matter into the entity's pit, fulfilling his part of the bargain.
Sorenson is returned, and the ship is allowed to leave. Sorenson has amnesia, and the Doctor suggests that he derives power from planetary motion.