The TARDIS arrives in the marshes of the third moon of the planet Delta Magna. While trying to get an accurate reading on the tracer, the Doctor is shot at by Thawn and Fenner, two men from a nearby methane refinery.
Romana is meanwhile taken prisoner by the green-skinned natives, the Swampies, who decide to sacrifice her to their god Kroll as a prelude to an attack on the refinery.
A man named Rohm-Dutt is running guns to the Swampies ostensibly on behalf of an anti-colonial organisation sympathetic to their cause. This however is part of a plot by Thawn to have the Swampies wiped out. The Doctor arrives at the Swampies' temple just in time to rescue his companion.
The refinery crew notice that the lake bed is shifting, as if there is something moving beneath it. The Swampies, led by Ranquin, prepare their attack, but are stopped in their tracks when Kroll - an enormous squid-like creature - appears on the horizon. Kroll starts to assail them, while the Doctor, Romana and Rohm-Dutt are captured by the Swampies and sentenced to death. The Doctor and Romana manage to escape, but Rohm-Dutt is seized and killed by one of Kroll's tentacles.
The two Time Lords reach the refinery, where they learn that Thawn is planning to destroy Kroll by blasting it with a rocket. The Doctor disables the rocket and Thawn is slain by the Swampies as they launch their attack.
Kroll then returns but the Doctor, realising that it has ingested the fifth segment (disguised as a Swampie relic), uses the tracer to end its threat. The segment is converted, and in place of the giant Kroll there are countless smaller creatures left lying in the marshes.
Has Prerequisite Story
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say this story requires a previous story.