Infection from a spider bite causes Romana's personality to change. Meanwhile the Doctor, after discovering that the spaceship society is in a perpetual state of repair and upgrade, has morality issues with the ship's leading scientist who readies himself to perform gruesome experiments on the live marshchild while it's fully conscious.

The Doctor and Romana find an isolated planet. K9 scans the planet, finding it has a village. The Doctor and Romana find high technology readings with the TARDIS sensors even though the village appears to have an absence of technology. The TARDIS lands near the village. The Doctor and Romana set off, leaving K9 behind to guard the TARDIS. Adric, from the previous story, has stowed away on the TARDIS. K9 finds him, but Adric tells him that as he is a stowaway. The Doctor and Romana find two men arguing. The Doctor asks to speak to a scientist. One of the men mistakes them for lords. After the Doctor, Romana and one of the men leave, The other man, using a radio, calls to others and tells them of the two strangers who just arrived. Travelling through the forest, Romana and the Doctor are surrounded by men. They surrender and are taken to a hidden cave, where they meet the rebels. One man is trying to get a computer up and running. The Doctor helps him and Romana identifies the computer system as coming from Earth, Adric tries to steal some food in the village. Adric is caught and those who caught him take on pity him and offer him a place to sleep because of the evil things that happen at night. The Doctor and Romana appear to be attacked by a swarm of bats at the end.