The Doctor is an observer and UNIT are providing security cover at an experimental drilling project designed to penetrate the Earth's crust and release a previously-untapped source of energy, named Stahlman's Gas after its discoverer. Professor Stahlman dismisses the concerns of the project's Executive Director Sir Keith Gold and exceeds all safety margins in order to expedite the work.

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 Doctor is accidentally transported by the partially-repaired TARDIS control console into a parallel universe where England is ruled by a military dictatorship.

The drilling project is at a more advanced stage here and, thwarted by his friends' ruthless alter egos, he is unable to prevent the penetration of the Earth's crust, which ultimately causes the planet's destruction. Escaping back to his own universe, where the drilling is still in progress, the Doctor tries to warn of impending disaster. At first he is disbelieved, but his words are borne out when the power-crazed Stahlman is himself transformed into one of the hideous primordial creatures.

The Doctor, aided by consultant Greg Sutton, kills Stahlman with ice-cold blasts from fire extinguishers. He is finally able to shut down the drilling with only moments to spare.