The Master, posing as a rural vicar, summons a cloven-hoofed demon in a church crypt. Seeking to gain the ancient titan's demonic power, he gathers a cult and then corrupts or controls the residents of Devil's End to bow to his will. Dark elemental forces begin to disturb the village on the eve of May Day: unexplained murders, a stone gargoyle come to life, and a nigh-impenetrable infernal energy dome. With the Master fully prepared to destroy the Earth, the Doctor and UNIT - aided by a benevolent practitioner of witchcraft - battle the wicked rites of a secret science weilded by an alien from another world.