"Colonel White, Captain Scarlet and Captain travel to an African game reserve to attend a secret conference that is being held underground in a room beneath a hunting lodge. The conference host, Spectrum Intelligence scientist Dr Giadello, unveils two anti-Mysteron devices inspired by the discoveries that Spectrum made when the Mysterons tried to assassinate General Tiempo. The "Mysteron Gun" fires an electron beam that permanently destroys Mysteron reconstructions by exploiting their vulnerability to electricity, while the "Mysteron Detector" is a radiographic device that identifies reconstructions through their resistance to X-rays.
Unknown to the delegates, Captain Indigo, a Spectrum officer working undercover as the lodge's waiter, is murdered by Captain Black and reconstructed in the service of the Mysterons. When the Mysteron Detector reveals his true nature, Indigo's reconstruction sabotages the conference by activating the controls that cause the lodge to descend into the ground. He then escapes in a car with the key to the controls, leaving the delegates to be crushed by the building above them.
Scarlet arms himself with the Mysteron Gun and escapes the conference room in a lift to confront Indigo. The lift is rendered inoperable when the lodge begins to descend, eliminating the other delegates' only means of escape and forcing Scarlet to pursue Indigo in a second car to recover the key. In the conference room, Blue slows the lodge's descent by firing his gun into the ceiling at the point where the controls are located, damaging them.
Indigo is intercepted by Scarlet and shot by one of the reserve's game wardens. However, the wound is not fatal, and Scarlet dispatches him using the Mysteron Gun. Speeding back to the lodge with the key, Scarlet reverses the building's descent moments before the delegates are killed. Later, all present agree that the Mysteron Gun and Detector have proven their worth in the field."
The episode refers to Episode 6, "Operation Time", describing the downfall and death of the villain, and how Spectrum learned from it how to combat Mysterons better. Unfortunately, while the Mysteron Detector is referred to and used many more times in the series, this episode marks the ONLY appearance of the Mysteron Gun. Scarlet even says that it's the only gun that can kill a Mysteron, despite he and other Spectrum operatives using ordinary guns to kill Mysterons later on.