Larry, a foreign newspaper correspondent, is on assignment in Japan. His editor sends him to do a piece on the work of Dr. Suzuki, a scientist conducting mysterious research in a converted home located on the slope of an active volcano. The editor figures that any work important enough to risk being in the path of an eruption should make for good news. He doesn't know the half of it. Turns out Dr. Suzuki has turned his brother into a yeti and his wife into a monster whose skin is slowly sloughing off. (!!!!!!!!) Suzuki also has a European assistant named Tara, who probably would have gotten turned into a monster, too, if not for Larry showing up. Dr. Suzuki sedates Larry with some doped saki, then injects him with "enzymes".
Over the next week or so, Larry turns into a drunken, horny bastard. Oh, and his right arm (where Suzuki injected him) starts acting almost as if it were an independent organism. Hmm. Larry starts his descent in debauchery with gang-bangs with geishas and week-long benders, then starts an affair with Tara. Larry's wife, concerned about the lack of contact with him, flies over to Japan and confronts Larry in his hotel room ... where he's just brought Tara. Larry makes a big show of choosing Tara over his wife, and they leave for her place. Tara, however, tells Larry that he owes it to his wife to have it out with her. He goes back to his room, where his wife has fallen asleep while waiting for him. As he reaches out to wake her (with his right arm), his hand turns furry and clawed! Larry chases his wife away, then goes out for a walk, obviously with a lot on his mind. He enters a Buddhist temple and tries to confess to the priest he finds there, but gets angry because the priest doesn't speak English and can't understand him. Next thing you know, a spinning newspaper headline declares, "TemplePriestFoundMurderedbyFiend!" (sic)
It only gets worse from there. He drinks so much it's almost like he needs the alcohol to live. A wound on Larry's right shoulder bursts open to reveal an eye! (!!!!!!!!!) Larry starts having Jekyll-and-Hyde blackouts every night, wherein he wanders the streets and murders anyone he comes across (usually a sexy young woman). Turns out it isn't just an eye under Larry's skin, but an entire head, as well! The head emerges, by the way, right when Larry decides to see a psychiatrist about his deteriorating mental condition. The shrink manages to call the cops before Two-Headed-Larry goes berserk and kills him, leading to a surprisingly well-done foot chase across the city.
Larry ends up at Suzuki's laboratory, where the doctor and Tara have been waiting for him. Suzuki is all like, "Gee, I'm sorry. I only wanted to better mankind by turning you, my brother, and my wife into monsters." Fortunately, Suzuki has a solution: another injection, which should speed along the development of the monster inside Larry, which will cause it to burst forth from inside him. How that won't be fatal to Larry and will, instead, "cure" him, I'm sure I don't know. Suzuki tries to commit seppuku, but Monster Larry is all like, "Naw," and kills him personally. He then carries Tara up the mountain, to the rim of the volcano. The air temperature at the rim is so high it incubates the monster, causing it to break free of Larry almost immediately. It wrestles with Larry, then picks up Tara and throws her into the volcano. While it's busy watching her fall, Larry pushes the monster in after her.
The police finally arrive, with Larry's wife in tow. Larry is presumably all kinds of screwed at this point, but the movie is unconcerned with this. The End.
They don't make 'em like this anymore: straight to the damned point. That's a shame. If this movie were made today, it would be three hours long, plod along with no sense of pacing whatsoever, have about 100 characters who don't do anything, 200 subplots that don't go anywhere, the action would stop every few minutes so someone could pontificate at length about the theme, and Dr. Suzuki would have those damned floating holograms for a computer interface in his laboratory. Fortunately, The Manster wasn't made today, so it can movie swiftly, have only as many characters and scenes as it needs, and everything we need to know can be communicated by the action instead of crushing blocks of dialogue.