Kyosuke Shikijo, the hapless college student who transforms into the perverted superhero Hentai Kamen by placing a woman’s used panty over his face, faces his greatest challenge yet. Now attending university with his longtime love interest Aiko Himeno, Kyosuke has promised her he will retire his unusual alter ego. To ensure he keeps his word, Aiko demands the return of her treasured panties, which have long been his primary power source. Reluctantly, Kyosuke agrees, leaving him powerless and attempting to live a normal life.

His resolve is immediately tested when a bizarre crisis grips Japan: all the women's panties in the country begin mysteriously vanishing. Kyosuke soon discovers that his old nemesis, Tamao Ogane, survived his apparent explosive demise, though now only his severed head remains. Operating from a hidden lair, Tamao seeks revenge by eliminating Hentai Kamen’s only weakness and power source. His plan involves stealing every pair of panties so Kyosuke has nothing to transform with.

Adding to the chaos is the mysterious new biology teacher, Professor Ayata, who takes an unusual romantic interest in Kyosuke. Her advances create a love triangle that further complicates his already strained relationship with Aiko. Meanwhile, Tamao deploys an agent known as Mr. Vacuum, who possesses a backpack capable of remotely sucking the underwear off any woman. After Kyosuke is nearly beaten while unable to transform, he is forced to break his promise, using a pair of Ayata’s panties to become Hentai Kamen once more. However, using this cursed source of power leaves him severely weakened.

The situation escalates when Tamao uses a mind-control device to enslave Kyosuke’s lovesick classmate, Makoto, transforming him into a powerful cybernetic mutant named Dynoson. Fused with guitar, king crab, and Dyson vacuum parts, Dynoson kidnaps Ayata and accelerates the panty theft, pushing Kyosuke to his limits. After a desperate battle in New York City where Aiko is taken hostage, a weakened Kyosuke seeks guidance from the spirit of his deceased father. He learns he must become an even greater "pervert of justice" and seeks out a reclusive master known as the Great Legendary Pervert for training.

The master, who is revealed to be Kyosuke’s own grandfather, teaches him to become a "stoic pervert." This new level of enlightenment allows Kyosuke to transform using fresh, unworn panties, a previously impossible feat. Armed with this new power and a long-nosed red demon mask that enhances his abilities, Hentai Kamen storms Tamao’s lair. In the climactic battle, it is revealed that Professor Ayata has been a spy for Tamao all along. Kyosuke is nearly killed, but Aiko’s enduring faith in him manifests in a fresh pair of her panties, which she places on his face. Supercharged by the pure power of Aiko’s love, Hentai Kamen rises to finally destroy Tamao once and for all.