A nuclear powered cyclotron facility in some caves under a moorland is experiencing mysterious power drains and mental breakdowns amongst staff. The Third Doctor and Liz meet Brigadier Lethbridge-Stewart there to investigate. A worker who was potholing in the caves is found dead with giant claw marks on his body, and his companion has been traumatised. Lawrence, the Director, resents UNIT's presence and feels that it will interfere with the working of the plant. Major Baker, the security chief, believes there is a saboteur in the centre, and the Doctor discovers that the logs of the nuclear reactor's operation have been tampered with. When the Doctor makes his way into the caves, he is attacked by a dinosaur-like creature before it is called off by a strange whistling sound.