From The Grindhouse Cinema Database
Reporter Larry Stanford (Peter Dyneley) is about to have his life changed for the worse after he is sent to get the scoop on what Dr. Robert Suzuki (Tetsu Nakamura) is doing up there in his mountain laboratory with only his assistant Tara (Terri Zimmern) to keep him company. Unfortunately for Larry, what the good doctor is doing is injecting people with an enzyme he's discovered so he can help advance mankind onto a newer plane of existence. The plane of real ugly, real murderous monsters. It seems that Dr. Suzuki has deduced that by combining a certain compound with cosmic rays - yup, that old chestnut - he can change a human being completely, it's just that the side effect of this is that the person in question goes on a killing spree that would put Freddy Kruger to shame. After offering Old Larry a drink that leaves him drugged to the eyeballs and asleep on the sofa, Doc Suzuki injects him with his serum - not a euphemism - and sets about recording Larry's descent into madness, murder, and turning into someone with two heads. Both of which are vugly as all hell. His wife and his boss try to save him from himself - and a steamy affair with Tara because they're party poopers and Larry has the small problem of being married - but will their efforts be all in vain as The Manster takes full control?
For a movie that clocks in at around 72 minutes, The Manster does pack a lot of action, suspense, and moral warning into its short runtime. Poor Larry is a man trapped in a life, not of his making. He took the job as a foreign correspondent because it paid well and he wanted to see the world, yet he's hardly seen any of it due to being moved from place to place to cover whatever news is breaking. He's married to a woman who he's barely seen in many years, and he's being moved off of his beat to sit behind a nice, cushy desk, seemingly, against his will. So when he gets the chance to taste one slice of freedom - under the guidance of Dr. Susuki - he throws himself into it with such passion and gusto that it's admirable. He doesn't know that he's being manipulated and that he's changing into a monster, all he sees is he can get hammered whenever he wants, chase all the ladies he can, and hook up with Tara while blowing off his wife. Which he does quite brilliantly when she shows up in Tokyo and makes him choose between the two women in his life. He also sure as sh*t didn't ask to be injected with the serum that is causing an almost irreversible change.
The Monster/Manster - see what they did there - that Larry eventually becomes is quite amusing to look at. A two-headed beast that seems about as frightening as a baby rabbit in socks, I think that it probably carried/carries more gravitas in the original black and white print than it does in the incredibly badly done colorized version, though even then I'm guessing it's still pretty funny. Yet, before he changes into a nightmare version of The Chuckle Brothers, there are some moments where the special effects look pretty damn good. Such as the scene where Larry discovers he has an eyeball growing out of his shoulder blade. Sure enough, it's not on the level of - say - An American Werewolf in London SFX, but it's easily as good as the Ash Campbell duel-headed scene from Army of Darkness. In fact, I wouldn't be surprised if The Manster was an inspiration for that whole part of the film.
Overall, I rather enjoyed The Manster. It's easy enough to follow, doesn't outstay its welcome, and its moral tale of not messing around with things we don't fully understand is a standard enough trope that the whole film will feel familiar to you, even if you've never seen it before. It's not the best way you could spend an hour and ten minutes of your life, but it sure as hell isn't the worst.