Mutsumi Narita is a high school student exhibiting typical adolescent tendencies, including a habit of peeping on female classmates, which earns disapproval from his childhood friend Megumi Iijima. His outwardly frivolous behavior conceals underlying determination and intelligence. These traits emerge when female students, including his crush Maiko Saotome, begin vanishing mysteriously. Narita partners with Iijima after she reveals herself as a time traveler from 2112 to investigate the abductions.

His personal connection to Saotome drives him to uncover the threat: replinoid monsters, alien entities that drain victims' bodily fluids. Narita undergoes significant change when Iijima selects him to wield a futuristic sword. He transforms from a bystander into an active combatant against the replinoids, facing physical dangers and confronting the horror of classmates being attacked or killed.

Narita's investigation exposes the replinoids' infiltration of his school and their link to the drama club. Discovering Saotome's abduction intensifies his resolve, shifting his focus from personal infatuation to a broader mission of protecting others. The narrative concludes ambiguously, with Saotome's fate unresolved and Narita continuing the fight against the replinoids alongside Iijima.