An American karate student in Japan searches for the sister of his friend and mentor.

G-Man Ted Riley is ordered to investigate happenings at Diamond Island, where a bogus Major Gray is reported engaged in manufacturing a new brand of secret gas for his own purposes. Riley blows up his motor boat just off the island and is picked up by Gray's men. On the island he discovers chemist Professor Baker (John Cowell) and his daughter, Anne, are held captive by Major Gray.