The TARDIS picks up a distress call. The Doctor and Sarah Jane Smith land on the planet Zeta Minor, near the end of the universe. They discover that a Morestran Geological Expedition has been attacked by an unseen killer and only Professor Sorenson (Frederick Jaeger) has survived. A military mission from Morestra has also arrived to investigate. Naturally, they assume the Doctor and Sarah are responsible for it all, but the killer is revealed to be a creature from the antimatter universe. Prof. Sorenson had removed some antimatter samples from the pit which is the interface between the two universes, and the creature was retaliating.
The Morestrians take off in their ship, but it is dragged back to the planet because of the antimatter Sorenson has brought aboard. The Doctor goes to speak to the antimatter creature, but he falls into the void. (He was not wearing his scarf because that was too difficult for the special effects.) He crawls out, but takeoff is still impossible. The Doctor believes it is because of the antimatter on his person, which had saved his life in the pit. He gets rid of it along with the rest, they think, but Sorenson has hidden samples on board, convinced he has found the perfect power source. Sorenson becomes infected by them and transforms into an antiman—a monster able to drain the life from others.
Salamar (Prentis Handcock) the Morestrion Commander, is becoming unhinged, to the point of nearly ejecting the Doctor and Sarah into space. He attacks Sorenson with a radiation source, but that only creates more Sorensons—invisible, glowing red, and deadly—which overrun the ship. The Doctor finds the original Sorenson, takes him back to the planet in the TARDIS, and throws him into the pit, along with all the antimatter samples. The antimatter creature is satisfied and returns Sorenson unharmed.
The story was heavily influenced by Forbidden Planet. The antimatter monster (Mike Lee Lane) is sometimes invisible and sometimes outlined in glowing red, like the Creature from the Id in that movie. There were two impressive monsters and the planet itself was conscious. An elaborate jungle-set was built by designer Roger Murray-Leach, which ended up in the BBC training manual as an example of excellent set design. It really was impressive and became a kind of character in the story.