The Doctor is taken to the Sea Devils' underwater colony where he tries to make peace with the creatures, only for the navy to attack the base.

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.