When a mechanical replica of King Kong is unable to dig for the highly radioactive Element X at the North Pole, the evil Doctor Who and his sponsor Madame Piranha (Madame X in the American release) decide to kidnap the real Kong. As an insurance policy they kidnap Lt. Susan Miller as well as her boyfriend Lt. CommanderJiro Nomura and Cmdr. Carl Nelson (Kong developed a crush on Susan when she Nomura and Nelson visited the Kong at his home on Mondo Island). Kong later escapes and heads for Tokyo. Susan and the others also escape. Who and his minions follow him and activate Mechani-Kong in order to recapture him. Susan is then grabbed by the robot and is taken to the top of Tokyo Tower and a battle ensues between Kong and his robotic replica.