Another adventure for everyone's favourite Time Lord. Still chafing at his exile to the planet Earth, the Doctor (Jon Pertwee) is called in to observe the Inferno drilling experiment of Professor Stahlman (Olaf Pooley). Stahlman's search for a new form of energy goes horribly wrong when a green slime emitted from the drill site transforms any who come into contact with it into primeval monsters. Meanwhile, after experimenting with the TARDIS console, the Doctor finds himself transported sideways in time to a parallel version of the Earth, where the Brigadier (Nicholas Courtney) is an eyepatch-wearing fascist and Stahlman's experiments threaten to destroy the world.