An anachronistic twelve-million-year-old human skull has been discovered by archaeologists and is now being used by Professor Fendelman in his time scanner experiments at Fetch Priory in contemporary England. The skull is actually an artifact of the Fendahl, an ancient creature that feeds on the life force of others.
Drawn by the operation of the scanner, the Fourth Doctor and Leela arrive as the experiments reach a peak. The skull is exerting an influence over the mind of Thea Ransome, one of the scientists in Fendelman's team, and glowing with power each time the scanner is activated. Thea is eventually transformed into the Fendahl core, and a group of acolytes assembled by Maximillian Stael - another of Fendelman's team, who is trying to harness the creature's power for his own ends - are converted into snake-like Fendahleen.
The Doctor shows the remaining scientist, Adam Colby, and two locals, Martha Tyler and her son Jack, how to defend themselves against the Fendahleen using rock salt. By activating the scanner once more, he triggers an implosion that destroys both the Priory and the Fendahl core.