When the sleepy town of Weyharrow is enveloped by a mysterious fog, the inhabitants find themselves behaving in strange and dangerous ways. Dr Steven Glaze, a young probationary GP, prescribes a most unorthodox treatment for arthritis; the vicar proclaims in morning service that the villagers are in the hands of the devil; and Phyllis Knabbe tragically commits suicide. Throughout the village people have seemingly taken leave of their senses.
Soon word leaks out and Weharrow becomes inundated both by the national press and a bus load of hippies seeking a magical experience, who believe that a nearby ancient pagan temple is somehow responsible for this strange phenomenon. But Steven Glaze and Jenny, a reporter for the local newspaper, feel sure that there is more to this than meets the eye and they set out to discover the cause - supernatural or otherwise - of everyone's drastically altered behaviour.