On Brigadier Lethbridge-Stewart's orders, Major Whitaker and her UNIT team inspect caverns with Dr Iskander to find the cause of sudden seismic activity. They find Dominator Haag and his Quarks on a quest to use the Earth's core as a power source for the Dominators' star fleet. Using the Dominators' own equipment, Dr Iskander and the UNIT team ensure the spacecraft, the Dominator and the Quarks are destroyed in a volcanic eruption.