Written by: Terry Nation
Directed by: Barry Letts
The Doctor and Sarah arrive in what they initially believe is the quiet English village of Devesham, located near Britain's Space Defence Station. Encountering many strange sights, including villagers seemingly entranced, they soon uncover that the village is in fact a replica built on the planet Oseidon. An alien race known as the Kraals, led by the scientist Styggron, have built a copy of both village and station and populated them with android replicas in order to prepare for their invasion of Earth. The Doctor and Sarah meet sinister versions of UNIT friends such as Harry Sullivan and Benton, while attempting to uncover Styggron's full plan.
Aiding the Kraals is an Earth astronaut named Guy Crayford, thought lost by Earth years ago during a space mission. Styggron has duped the man into helping them, after supposedly finding his damaged space craft and repairing Crayford's injured body. The Doctor and Sarah eventually manage to return to Earth in Crayford's rocket, as his miraculous return is part of the invasion plans.The rocket also contains a deadly virus that Styggron will release on Earth to weaken the population ahead of a full invasion. A convenient meteor shower is also part of the plan, as the meteors contain android duplicates that will take over the Station.
Though the Doctor and Sarah's warnings fall on deaf ears due to the joy at Crayford's return, they do manage to use the Station's radio transmitters to jam the control signals of the androids and eventually succeed in preventing Styggron from releasing the virus. Styggron himself is killed after he is confronted by Crayford, who has discovered his only an android himself, and the alien scientist is felled by his own virus.