The Doctor and Romana land on the moon Delta III, home to the green-skinned Swampies and a small group of human colonists. Soon after arriving, Romana is taken by the Swampies who wish to sacrifice her to the mysterious squid-like Kroll.

The Doctor, Romana and Rohm-Dutt are sentenced to death by the Swampies while the refinery crew try to work out a way to destroy Kroll.