Jo and the doctor get caught up in a series of terrifying events surrounding the secret of an ancient burial ground. For 200 years the secret of Devil's End village has lain dormant, as the burial mound, the so-called Devil's Hump, has remained sealed. Until now, at midnight on April 30th on the greatest occult festival of the year, as Professor Horner prepares to break it open. But just what evil treasures lie buried there? Will the local white witch's warnings of doom and destruction be fulfilled? Who or what is the mighty Azal? But even more disturbing for the Doctor and Jo, just how is the Reverend Magister, alias the infamous Master, involved in the mystery?