A 12 million-year-old human skull has been discovered by archaeologists and is being used by Professor Fendelman in his Time Scanner experiments at Fetch Priory on contemporary Earth. Drawn by the operation of the Scanner, the Doctor arrives as the experiments reach a peak. A strange force is called into existence when the Scanner operates, and a hiker has been killed in the woods as well as a security guard at the house which is being used as a base by the scientists.

One of the scientists, Thea Ransome, is also affected. The skull is exerting an influence over her mind and each time the Scanner operates, the skull glows with power as the connection is strengthened.

The Doctor realises that the skull is a channel through which a powerful and ancient creature called the Fendahl will manifest on Earth. The Fendahl lives by sucking the life from others and it had been thought destroyed by the Time Lords when, following its destruction of life on Mars, it was trapped on the fifth planet in the solar system which disappeared when the Time Lords threw it into a time loop. Since then the creature has been subtly affecting and influencing life on Earth, guiding it to a point where the Fendahl could manifest. It is no coincidence that the scientist in charge of the Time Scanner is called Fendelman, nor that Thea is sensitive to the ancient skull. This is all a part of the Fendahl's plan.

Thea is transformed into the Fendahl core, and a group of acolytes assembled by Maximillian Stael, another scientist who is trying to harness the power of the Fendahl, are all converted into snake-like Fendahleen. The Doctor organises the remaining scientist, Colby, together with a local wise-woman Granny Tyler and her son, Jack, to defend themselves against the Fendahleen using rock salt loaded into shotgun cartridges. The Doctor then takes the skull back to the TARDIS as the Time Scanner is operated for a final time. The resultant power surge destroys the house and the Fendahl Core, while the Doctor aims to dump the skull into a super nova in space and thus destroy the Fendahl's bridgehead.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telo}s