The Doctor becomes alarmed on seeing television coverage of an archaeological dig by a Professor Horner into an ancient barrow near the village of Devil's End. He hurries to the scene with Jo.

The Master is posing as the local vicar, Mr Magister, and using black magic rituals to summon Azal, the last of a race known as the Dæmons, whose miniaturised spaceship is buried within the barrow. Benton and Yates arrive in a UNIT helicopter but, before the Brigadier and his troops can join them, a heat barrier appears and cuts the village off from the outside world.

Azal will appear three times and on the last of these occasions will decide whether to transfer his awesome powers to another or to destroy the planet as a failed experiment. The Master hopes to be the recipient of the powers, but in the event Azal offers them to the Doctor instead. The Doctor declines, arguing that the human race should be allowed to develop at its own pace.

Azal decides to kill him, but Jo then offers to take his place and, unable to comprehend this act of self-sacrifice, the Dæmon self-destructs. The Master is finally captured by UNIT and taken away to await trial for his crimes against humanity.