A scientist develops the means to advance the evolution of a man by 20,000 years. The process then continues beyond his control. The man who is evolving encounters police officers and says to them: "Your ignorance makes me ill and angry."

Working in a remote Welsh mining town, a rogue scientist, Professor Mathers (Edward Mulhare), 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. A disgruntled miner, Gwyllm Griffiths (David McCallum), volunteers for the experiment, enabling the professor to create a being with enhanced mental capabilities. 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. When the mutation process begins to operate independently of the professor's influence, Gwyllm takes control of the experiment. Now equal to 1 million years of evolution, and equipped with superior intelligence and powers of thought that are capable of great destruction such as telekinesis, Gwyllm seeks vengeance on the mining town he loathes. Before he acts on his thoughts, Gwyllm holds back. He has evolved beyond concepts such as love, hate, and the desire for power, and instead intends to turn himself into a vortex of pure intellect with the help of his girlfriend, Cathy Evans (Jill Haworth). Out of love for him, however, Cathy reverses the process at the last second, bringing Gwyllm back to his former self. But, the out-of-control reversal is too much for Gwyllm, and he slowly succumbs to the adverse effects while Cathy comforts him.

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. The start of Act Four, showing Gwyllm's vengeful rampage against the mine, was removed. Originally, Gwyllm kills Wilks and his deputies, then proceeds on to the mine. The gatekeeper sees him coming, enveloped in a bituminous aura. Mr. Caradoc sets off the disaster whistle just as Gwyllm reduces him to smoldering ashes. Gwyllm then places a box of dynamite at the pit entrance, igniting the fuse with a burning glance. Emlyn, one of the trapped terrified miners, attacks him with a pickaxe and is flattened by a burst of kinetic force. Gwyllm then becomes translucent, evolving beyond the need for vengeance and abandons the burning fuse to walk back to Mathers' Lab. Robbart, another miner, stamps out the fuse. Gwyllm later tells Cathy his ghostly translucence is evidence of his further evolution: "I can now live by photosynthesis of pure light." The dialogue between Gwyllm and Prof. Mathers at the end of Act Three, when Gwyllm declares his intentions to obliterate the village, was originally slightly longer. This shortened version of the script was only 40 pages long (one page roughly equaling one minute of screentime). To fill the gap, the sequence of Gwyllm discovering music was quickly written as a five-minute insert by Joseph Stefano. For this, the then-new Glenn Gould recordings of the Bach preludes were used, which were performed faster than anyone had done them before. The scene used three different preludes. As it opens, Gwyllm plays the Prelude and Fugue No. 2 in C Minor. Most of the conversation with Mathers uses Prelude and Fugue No. 5 in D Major. When he says "I shall stop soon anyway," he begins Prelude and Fugue No. 1 in C Major.

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