Doctor Phil Bogart is killed by the mysterons whilst he is rock climbing in Arizona, he is replaced by a mysteron replicant who steals a ‘Thunderpulse’ bomb. It soon becomes a race against time to find the 'Thunderpulse' bomb that Phil has hidden in a major city. When Spectrum descovers what has happend Colonel White informs Scarlet and Blue that Phil has a twin brother, Frank, who Phil shares a phic link with but will he help them?
