Still weak and confused after his fourth regeneration, the Doctor retreats to Castrovalva to recuperate. But Castrovalva is not the haven of peace and tranquility the Doctor and his companions are seeking. Far from being able to rest quietly, the unsuspecting time-travellers are caught up once again in the evil machinations of the Master. Only an act of supreme self-sacrifice will enable them to escape the maniacal lunacy of the renegade Time Lord.