On the eve of May Day, dark elemental forces begin to disturb the village of Devil's End as the Master summons the demon Azal: 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 wielded by an alien from another world.

A warp ellipse draws the TARDIS off course. The Fifth Doctor's companions are separated from him not in space, but in time, and he has to deal with a treacherous schoolboy named Turlough. But why does the Doctor's old friend, the Brigadier, not remember him at all?