In a nutshell: An American reporter in Japan encounters an unscrupulous scientist who uses him as a guinea pig for experiments involving evolution, cosmic rays, and shitloads of alcohol. The reporter begins sneaking around Tokyo at night to commit murders and eventually grows an extra head that separates into a hairy "new species" ape monster.
Can you stand the sheer terror???
THE MANSTER is another flick that I saw during my formative years when it ran on Channel 5's iteration of CREATURE FEATURES., and I mostly watched it because it was a shot-in-Japan American production that looked and felt just like the Japanese films that I learned early to favor. Though not a city-stomping giant monster opus, it was worth watching to see how Americans handled Japanese-style scares, but it comes off like a monster movie as written and directed by a nine-year-old. The story's stupid as hell, not at all scary, and rather mediocre, so why am I bothering to call your attention to it? For one reason: the indelible and genuinely creepy moment when the reporter discovers an eye generating on his shoulder.
The best moment in an otherwise idiotic movie.
THE MANSTER is by no means a classic, but it's a fun way to kill 72 minutes and it has the decency to not be boring.
Poster from the Japanese theatrical release.