UNIT is providing security cover at an experimental drilling project designed to penetrate the Earth’s crust and release a previously untapped source of energy. Soon, however, the drill head starts to leak an oily green liquid that transforms those who touch it into vicious primeval creatures with a craving for heat.

The Third Doctor is accidentally transported by the partially repaired TARDIS control console into a parallel universe where the drilling project is at a more advanced stage. Thwarted by his friends’ ruthless alter egos, he works to save both universes.