In the village of Fetchborough, four scientists have gathered at the Priory. They are Adam Colby (Edward Arthur), Max Stael (Scott Fredericks), Thea Ransome (Wanda Ventham), and Doctor Fendelman (Denis Lil). They are doing tests on a 12 million-year-old skull they found in Kenya. A sonic time scan used to get an image of the skull’s owner causes a reaction, in which something is released than kills a passing hiker and slowly disintegrates his body. It seemed particularly interested in Thea.
The scan attracts the attention of the Doctor and Leela. In fact, the TARDIS is pulled down to Earth by it. They need to find the source of the radiation before it causes a continuum explosion and destroys the planet. Leela finds the cottage of Mother Tyler, a local witch. The Doctor narrowly avoids being killed by the creature created by the skull, which then kills a guard Doctor Fendelman hired after the death of the hiker, which he is covering up.
Ma Tyler encounters the creature, but the Doctor saves her from psychic shock with a nice cup of tea. He knows now that it is a Fendahleen, a creature from Gallilean mythology. He gains access to the Priory and finds the skull, which tries to kill him. Leela saves him and they head off to the destroyed fifth planet of the Terran solar system, only to find that the Time Lords have sealed the place in a time-loop, making all records inaccessible.
Thea has been gradually converted into a Fendahleen golden goddess, who feeds off life energy. Stael, who is leader of the local black magic cult, believes he can control the creature. He and his followers capture Colby, kill Fendelman, and set up the sonic time-scanner to control the skull and Thea’s transformation.
The Doctor, Ma Tyler, and her grandson Jack (Geoffrey Hinsliff) head for the Priory, to find that the Fendahl core of twelve has formed and is converting the whole cult into Fendahleen. The place is overrun with larval Fendahleen monsters. The Doctor frees Colby, helps Stael shoot himself before the change is complete, and finds out that the Fendahleen are fatally allergic to salt, which they load into a shotgun. The Fendahleen are now two short of the twelve needed to form a circle. The Doctor rigs the scanner to implode, grabs the now powerless skull, and escapes with the others just before the implosion. The Doctor and Leela leave in the TARDIS to dump the skull near a supernova. This is another story influenced by Quatermass. Also, The Sirens of Titan by Kurt Vonnegut. It was heavily influenced by the work of archaeologist George Leakey. Colby’s dog is named Leakey in his honor, but this resulted in jokes that Tom Baker found particularly hard to resist. It was filmed on Mick Jagger’s Stargroves estate in Hampshire, which had also been used for Pyramids of Mars. Unfortunately, the film-shoot’s generator exploded and disturbed the neighbors. Wanda Ventham, who had lost out on Shirley Eaton’s role in Goldfinger, was excited to be turned into a golden goddess. She is Benedict Cumberbatch’s mother. Leela had a new sexier (!) outfit, but more comfortable. K9 had to stay in the TARDIS because he had come down with a case of circuitry corrosion. Already, they were beginning to discover what a pain the little robot could be.