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.
Ptela and Jo leave the ship and hide. Jo tells Ptela about her friend, the Doctor, who has been killed by the Thrematons. Ptela begins to fall in love with Jo. They climb one of the Thrematon ships and are captured. They are rescued by the Doctor, who explains that he is immune to the Thrematon mutagene. He destroys the Thrematon ships when they refuse to leave.
Ptela is injured, so they take him to the TARDIS. He wants Jo to stay but she won't, so he pretends to be infected by the Thrematon mutagene. He leaves the TARDIS, hoping Jo will follow. She and the Doctor follow Ptela to the edge of a volcanic crater. Ptela drags the Doctor into the crater, and the Doctor is submerged in the lava. Ptela tells Jo he loves her, but the Doctor appears, having been wearing a portable atmosphere device.
Jo makes her farewells to Ptela, and she and the Doctor leave him behind to help rebuild his world.
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