The Thrematons have invaded the Ancetetian colony, intending to terraform the planet. Several of their spaceships land on the planet and release a mutagene that transforms the Ancetetians. Ptela and his brother Roshon enter a spaceship and Roshon becomes infected. They come across Jo Grant, who is being chased by a transformed Ancetetian. Roshon sacrifices himself to saving Ptela and Jo. Jo makes her farewells to Ptela, and she and the Doctor leave him behind to help rebuild his world.