Kyosuke (Fukuda) is just your average, normal hormonally-driven teenage boy. Well, as "average" and "normal" as he could be, given he's the offspring of a masochistic police detective and a dominatrix. Through a set of circumstances which only ever happen in superhero movies like this one is parodying, he discovers that when he wears women's panties on his head, he transforms into "Hentao Kamen", which roughly (and accurately) translates as "Pervert Mask". This allows him to go around saving his pals from the likes of "Goody Two-Shoes Man", and other characters seeking to take over the school. His crush, Aiko (Shimizu), naturally falls for the hero without knowing his true identity, but trouble arises as a copy-cat Hentai Kamen shows up, without any of the moral compass which Kyosuke possesses. He's more than a match for the "good" HK, and soon the hero's reputation is in tatters. There's only one pair of panties which possibly contain enough power for Kyosuke to defeat his nemesis.
Definitely deserving to be filed in the "only the Japanese..." file, this is not as sleazy as you might imagine, with a hero that comes across as likeable rather than depraved. The results are probably all the better for it, going more for broad humour; if you think Carry On Spiderman, you're probably closer than you'd expect, though the production values are also better than I expected. Based on a long-running manga series, which provides ideas such as the "spinning crotch attack," this is at its best when throwing a barrage of ideas as the screen, and riffing on the basic concept. For, let's face it, the superhero whom, inexplicably, nobody recognizes is ripe for parody e.g. Superman puts on a pair of glasses and becomes Clark Kent. There isn't enough sense of progression here though, and less successful is the Big Bad, who never seems like much of a threat to anyone other than Kyosuke. I get the sense it might have worked better as a late-night television series, as some of the ideas here might merit broader development. Otherwise, find the trailer and you've more or less seen the movie.