In the Village of Devil’s End, archaeologists are digging up the Devil’s Hump, a Bronze Age burial ground. The local white witch, Olive Hawthorne (Damaris Hayman) warns of great evil and the coming of the Horned Beast, but nobody pays any attention. After seeing the dig on TV, the Doctor tells Jo that Miss Hawthorne is right, and the dig must be stopped. Miss Hawthorne goes to the new vicar, Reverend Magister, who is really the Master (Roger Delgado), and he tries to hypnotise her to keep her quiet, but he fails. Surrounded with followers, the Master is performing ceremonies in the Cavern beneath the Church. He wants to conjure up the Demon Azal (Stephen Thorne, who is six-foot-six, with a deep voice). The Doctor and Jo are too late. The tomb opens and icy gusts of wind rush out, nearly freezing the Doctor to death.
Captain Mike Yates (Richard Franklin) and Sergeant Benton (John Levere) of UNIT arrive the next morning, but the Brigadier (Nicholas Courtney), who arrives later, cannot enter the village because there is an invisible dome ten miles wide and one mile high surrounding the area. Anything that tries to penetrate the field bursts into flame. The Doctor and Jo return to the dig and find a spaceship inside. The Doctor realizes the Master is trying to summon an ancient Daemon, believed on Earth to be the Devil, but really an alien that has been interfering with the human species since the time of the Neanderthals. The Master succeeded in calling it up once, so small as to be invisible, but the next time could be the end of the world.
The Master summons Azal again and demands its power, but Azal warns him that he will not be the Master’s slave. On his third summoning he will decide if the world should be handed over to the Master or simply destroyed, then vanishes. The Doctor is captured by a village mob working for the Master. They tie him to a Maypole and plan to burn him alive, but with the help of Miss Hawthorne and Sergeant Benton, he escapes. In the Church, Jo and Captain Yates watch the third summoning. They try to stop it but are seized and taken prisoner. Jo is—of course—prepared for sacrifice.
The Brigadier manages to penetrate the heatshield with a device invented by the Doctor, but they are held off by an indestructible stone gargoyle with glowing red eyes, named Bok (Stanley Mason), though the Doctor manages to enter the Cavern. The Doctor and the Master appear before Azal, grown to giant size, who sides with the Master and fires electricity to kill the Doctor, but Jo steps in front of him and offers herself as a sacrifice. In the face of this, Azal’s powers are turned back upon him and he is destroyed—if you believe that. The church collapses and everyone runs for their lives. The Master is taken into custody by UNIT and driven off.
Jo Grant’s scene of self-sacrifice was Katy Manning’s audition scene for the role of companion, around which the story was written. It was perfectly in her character—good-hearted, passionate, and impulsive—and shows why she made such a good companion. Everyone was careful to downplay the Satanist tropes, but that was hard to do. The story was filmed in Aldbourne, Wiltshire, many of the villagers hired as extras. A friend of Damaris Hayman, who played Miss Hawthorne, was a genuine registered witch and both were advisors on supernatural matters. The scene in which the model of the church was blown up was so realistic that the BBC was deluged with angry letters about destroying an ancient monument. The shot of the Brigadier’s helicopter crashing into the dome was a From Russia with Love out-take. The Master’s incantation was Mary Had a Little Lamb backwards; they were going to use the Lord’s Prayer but thought better of it. This serial marked the end of Roger Delgado’s five-story Master Series, but he returned occasionally in the next two seasons.