UNIT have been assigned to provide security for an experimental operation, nicknamed the Inferno. The Doctor is also present as a UNIT observer but is secretly siphoning off power.
Contact has been lost with Mars Probe 7 since it set off on its return journey to Earth. The Doctor embarks on a solo mission the recovery ship but is intercepted by an alien ship.
The Doctor and UNIT are called in when an underground research center experiences problems. Nearby the Doctor discovers a group of intelligent reptiles, known as the Silurians.
Recovering from his regeneration, the Doctor is taken to hospital where his alien physiology is a puzzle. Meanwhile, the Nestene consciousness intend to conquer Earth using Autons.