The Doctor comes face to face with a Silurian and Major Baker, supposedly confined to the sick bay, heads off to find saboteurs in the caves. Liz and the Doctor find him captured and a doorway to the Silurian enclave.
Brigadier Lethbridge Stewart enlists the Doctor and Liz to investigate power failures at an underground atomic research center. There they learn that an incident has left one of their technicians dead and another frightened out of his wits.
The Doctor faces the end of his life as the Master's plans hurtle out of control. With the sound of drums growing louder, and an ancient trap closing around the Earth, the Doctor and Wilf must fight alone. But sacrifices must be made, and the deadly prophecy warns: "He will knock four times."
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