The Doctor is called to an underground facility where he learns that work on a nuclear reactor is being delayed by mysterious power losses.  The Doctor discovers a group of intelligent reptiles near the base that went into hibernation millions of years ago.  They were to have been revived long ago but their machinery broke down.  They have now been revived by power from the nuclear facility.  The Doctor attempts to make peace between the Silurians and humans but is thwarted when a rebellious Silurian releases a virus that may destroy out humanity.

The Doctor finds an antidote to the virus.  The Silurians take over the facility and attempt to destroy the Van Allen Belt.  This would be harmful to humans but would help the Silurians since they are reptiles.  The Doctor overloads the nuclear reactor and the Silurians return to their caves.  The Brigadier blows up the Silurian base against the Doctor's wishes.