Working in a remote Welsh mining town, a rogue scientist, Professor Mathers, discovers a process that affects the speed of evolutionary mutation.
Mathers suffers guilt for his role in developing a super-destructive atomic bomb, and hopes his new discovery will better the human race.
As a man sent forward equal to 20,000 years of evolution, Gwyllm soon begins growing an overdeveloped cortex and a sixth finger on each hand.
Now equal to 1 million years of evolution, and equipped with superior intelligence and powers of mind, such as telekinesis, that are capable of great destruction, Gwyllm seeks vengeance on the mining town which he loathes.
Regarding Ellis St. Joseph's original script, a number of scenes and characters were removed or condensed to save money, including five speaking parts – Bryn Evans, Gert the Bread's legless, Bible-thumping husband (Cathy Evans' father); Wilks, the local police constable; Robbart and Emlyn, two coal miners, and the fat Mr. Caradoc, the pit owner.
One of the earliest deletions from the script was a speech by Prof. Mathers on the subject of recapitulation theory: "In the short span of nine months, every human embryo passes through a million years of its previous evolution, from protoplasm to fish to amphibian to furry ape with a tail to man.
– the dialogue foreshadowing the fate Ellis St. Joseph had in store for Gwyllm once Cathy pulls the lever on Mathers' machine, reversing his evolution.
However, ABC executives would only allow him to regress to the stages of early man, due to concerns that religious viewers who did not accept the theory of evolution would be offended.