 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.