Deep space, an unknown time - on course for Gallifrey, the TARDIS passes through a strange disruption. When they materialise, the scanner shows the Time Lord homeworld... but the travellers have passed into the negative pocket universe of E-Space and are in fact on the planet Alzarius, where evolution works very differently...

As the humanoid community takes refuge in their crashed spaceship, venomous marsh spiders begin to hatch from the local fruit while bestial marshmen emerge from the waters to walk the land. It is quite apparent that the marshmen desire entry into the spaceship. Meanwhile, as a curious marshchild shadows the Doctor, a group of teen rebels decides to take over the TARDIS.

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.

Marshmen invade the Terradonian ship thanks to Romana, who seems to be in communion with them. While the ship's inhabitants flee for their lives, the Doctor discovers a crucial fact why these humanoids can never return to the planet Terradon.