On the planet Vortis, the Doctor and his companions aid the butterfly-like Menoptra in their battle against the parasitic Animus and its mind-controlled minions, the ant-like Zarbi.
The Doctor, Vicki and Steven arrive on an arid planet where they meet the beautiful Drahvins and the hideous Rills. Each has crash-landed after a confrontation in space. The Rills are friendly, compassionate explorers. The Drahvins are dull-witted, cloned soldiers, terrorised by the intelligent, warlike matriarch Maaga.
