The Doctor (Tom Baker) and Sarah Jane return for a visit to Earth, only to discover that a wave of evil Mandragora energy has hitched a ride on the TARDIS and is now being used to plunge mankind back into the dark age. They have to thwart not only the murderous count, but also prevent the mad monk Heironymous and the Cult of Demnos from unleashing the Mandragora Helix on mankind.