In the Scottish Highlands, an elderly lady named Mrs. Mackenzie is alarmed to see green lights hovering over the nearby Grampian Nuclear Power Station; she reports the incident to the authorities, but no one believes her. When Colonel White learns of the incident he immediately sends Captain Scarlet and Harmony Angel to investigate. But that turns out to be just what the Mysterons want as the Spectrum Rhino ground vehicle is just what they need to breach the reactor's defenses.