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.

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.