It's a light-hearted pink film with a no-name cast, even compared with other pink films. It's trash and it knows it. It's not even particularly good trash, to be honest, but it's capable of being amusing and I didn't mind it.
It's based on a manga about a lecherous schoolboy (Touru) who can turn invisible. He hasn't invented a serum or anything. His body just does it automatically whenever he eats salted salmon roe. Naturally he takes this opportunity to be a twat, overact and pull stupid faces. That's the first half of the film. There's skirt-flipping, panty shots and general silliness. Sometimes this crosses the line into "piss off" (e.g. an invisible Touru deciding to lick girls), but it's harmless.
After that, the second half involves two incompetent crooks (and briefly would-be rapists) who mistakenly kidnap a girl (Yoshie) who likes Touru.
The storyline's actually okay. Nothing special, but it does the job even if Touru's the world's biggest idiot in Part 2. Yoshie is in the hands of kidnappers. Somehow Touru discovers where they're keeping her, although the film never tells us how. Given this situation, what would you do? (a) go to the police. (b) turn invisible and sneak into the building. (c) brazenly walk in on the crooks, strip naked in front of them and only then try to turn invisible. "Hilariously", you then discover that you've left your salmon roe in your other trousers. The crooks tie you up.
Touru's boarding with an all-female family. Because the Japanese word for "sister" can be used generically for anyone female of the right age, for a while I didn't know whether or not he was related to them. I don't think he can be... or, rather, I really hope he isn't. Anyway, there's a cheeky youngest one (Rumi), an older one (Aiko), a pretty one who likes him (Yoshie) and a granny who made me laugh and doesn't get nearly enough screen time (Madoka). The family interactions are quite likeable and probably the film's best material, although that's not saying much. Oddly, the film never shows their naughty bits on the frequent occasions when they get undressed in the bathroom. (I say "oddly" because tits and arse are on full display elsewhere.)
The school scenes, conversely, tend to be annoying. Touru has two equally sex-crazed classmates, played by Yakan Nabe (born 1970) and Yoshifumi Ikeda (dunno when he was born, but I'd guess he's also in his forties). You've almost got to admire the film's brazenness. This goes beyond Hollywood casting. No one could have believed that this casting could convince anyone. It's absurd, perhaps as a deliberate joke.
There's also an annoying male teacher who's secretly also lascivious, played by Akitoshi Ohtaki, and an even more annoying sexpot female teacher (played by Haruna Ito) who can't be a day older than any of the actresses playing schoolgirls. She looks fifteen at most. She's also wooden and unconvincing when trying to play "sexy", even though that's pretty much the role's only requirement. The actress's age also damages the scene where Ohtaki (born in 1963) breaks into her home and starts trying to seduce her, which is incredibly creepy and awkward until you remember that Ito's playing a teacher. (After that, the scene becomes merely grating.)
This couldn't be called a good film, but it has its moments. It can be funny, e.g. granny, or some of the scenes where Touru's naked. (I laughed.) It also has CGI-assisted invisibility with some cool moments. I loved seeing our Invisible Man eat and drink. It's also silly in a whimsical way, e.g.
1. Touru bumps into the movie's logo when it appears on-screen.
2. The absurd crooks.
3. When someone does something particularly daft, the other characters might fall over.
4. The stuffed toy dog at the end. It's meant to be real, i.e. a live pet animal. I couldn't believe it when I worked that out. I couldn't understand what the film was trying to convey by having a fluffy toy attack Touru.
This is a quick, cheap film. Sometimes it's surprisingly amusing. Sometimes it's tiresome. It's too inconsistent to be worth recommending, even to someone looking for low-rent J-sleaze, but I sort of enjoyed it. Mitsuko Hoshi's great as granny. Its fundamental problem is that Touru isn't very likeable. He can be rude and unpleasant to Yoshie, e.g. booting her curtly out of his bedroom when she's trying to be nice. (He wanted to masturbate.) However he tries to help in the second half, while his relationship with Yoshie is basically romantic. (She likes him, but he's capable of being an idiot and/or a dick.)
I'm sure it's not the best Invisible Man sex comedy, though. They're everywhere, if you start looking for them...