The Doctor and Jo arrive on the planet Solos, on a assignment by the Time Lords, where they learn that the native Solonians are mutating into ant-like mutant creatures...

Ky transports himself and Jo down to Solos and the Marshal only agrees to help her if the Doctor assists Professor Jaeger in altering Solos' atmosphere.

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

The Doctor and his friends are led to safety by Professor Sondergaard, who tries to help the Doctor translate the tablets.

The Marshal's premature use of Jaeger's experiment is threatening to destroy Solos' atmosphere and the Doctor returns to Skybase to try and correct the damage.

The Doctor is forced to help the Marshal cover up his actions with the Investigator but Jo and the others attempt to escape to expose him.