An American reporter, Larry Stanford, who is working for the Far East Times, interviews a Japanese scientist who has new theories about evolution and mutation. He doesn't know it but the doctor is a "mad scientist". In fact, his wife had volunteered for an early experiment and now he keeps her like an animal in a cage. She is horribly deformed. The doctor gets the idea to experiment on this American and injects him with a serum while he's asleep. Then, he follows the guy back to Japan to make sure the experiment is working. It's during this time Larry decides to leave his wife and start dating the doctor's assistant, Tara. Slowly, the guy starts changing--and the first thing is a human eye that appears on his shoulder. Then, he grows another head that looks like a caveman's. This two head creature, like a two-headed Mr. Hyde, goes around and kills people. Eventually, though, events take him back to the lair of the mad scientist and the reporter and this "growth" split, so now there's two creatures-- one is him, the other is a hairy ape-man.
I remember being creeped out by this when I was a kid and some parts are still effective. However, the whole morality story this tries to end with is laughable. An interesting version of DR. JECKYL AND MR HYDE nevertheless...