A mysterious, unnatural yellow fog is released from a fissure in the ground following an earthquake, causing anyone who comes into contact with it to descend into uncontrollable, violent madness. The story follows John Holman, a government researcher, as he attempts to discover the source of the phenomenon and find a way to stop its rapid spread across the English countryside. As society collapses under the chaos of the infected population, Holman races against time to contain the deadly mist before it reaches major cities.