The Doctor and Jo arrive on the planet Solos, on a assignment by the Time Lords. Where the Doctor and Jo learn that the native Solonians are mutating into ant-like mutant creatures and they uncover a conspiracy, where the evil Marshal of the Earth Empire plots to rule Solos for himself and plans to convert Solos atmosphere, since the planet's atmosphere is poisonous, so the planet can be colonized by the humans and he plans to wipe out all the mutated Solonians, who are evolving into a advanced super race.

The Doctor and Varan travel down to Solos to search for Jo and Ky but the Marshal orders his men to pursue them.