The live-action story of HK/Hentai Kamen begins with Kyosuke Shikijo, a shy and awkward high school student who is secretly a skilled martial artist. Despite his physical abilities, he is painfully timid around girls and often gets beaten up by bullies because he cannot stand to witness an injustice. His life takes a chaotic turn when a beautiful new transfer student, Aiko Himeno, arrives at his school. Kyosuke immediately falls for her, but his chance to be a hero comes sooner than expected when Aiko is taken hostage during a botched bank robbery.

Unable to wait for the police, Kyosuke sneaks into the bank to rescue her. In a rush to disguise himself, he grabs what he thinks is a mask but is actually a pair of women's panties. The moment he pulls the underwear over his face, the fabric touching his nose triggers a bizarre physical transformation. Kyosuke unleashes hidden powers that come from his unusual lineage. His father was a detective with a strong sense of justice, while his mother, Maki Shikijo, was a professional dominatrix in an S&M club. This combination of heroic will and sadistic perversion gives him superhuman strength, agility, and the ability to use ropes and restraints as weapons. Wearing only the panties on his head, a pair of briefs over his shoulders, fishnet stockings, and sneakers, he christens himself Hentai Kamen, the Perverted Mask, and easily defeats the criminals to save Aiko.

The setting is a typical Japanese city and Kyosuke's high school, which becomes the primary battleground for his secret identity. Kyosuke must keep his identity a secret, especially from Aiko, who is horrified by the antics of Hentai Kamen but unaware he is the boy she is beginning to like. The primary antagonist is Tamao Ogane, the head of the school's karate club, who wants to destroy the school to unearth a buried treasure. Unable to defeat Hentai Kamen in a fair fight, Ogane sends a series of bizarre, themed assassins to eliminate him. These enemies include sticklers for rules like Goody Two Shoes Man and other eccentric fighters such as Manly Man and Pleasant Man.

A notable narrative arc involves the corruption of Hentai Kamen's image by his math teacher, Professor Towatari. In an attempt to discredit the hero, Towatari dons a fake version of the costume and commits petty crimes. Kyosuke must fight his own teacher to clear his name. The climax occurs when Ogane unleashes a giant mecha robot on the school. To gain enough power to stop it, Kyosuke must ask Aiko for a fresh pair of her panties to wear. Despite her deep embarrassment, she agrees, allowing Hentai Kamen to access his full potential and destroy the robot. The story resolves with Kyosuke continuing to protect the city, hoping that one day Aiko will accept both his normal self and his perverted heroic identity.

In the sequel, Hentai Kamen: Abnormal Crisis, the stakes rise when a villain known as Tadashi Makoto threatens to steal all the panties in the world. This act would not only disrupt society but also rob Kyosuke of the source of his power. The film explores a crisis where Kyosuke struggles to transform without his required "mask," forcing him to confront his identity and seek the help of a reclusive perverted hermit master to unlock new abilities.