Kenta Kogure, an outwardly average Japanese high school student, harbors romantic feelings for his bright red-haired classmate Kotomi Asakawa. He remains oblivious to classmate Rika Akagawa's crush on him while maintaining a close friendship with Jinpachi Shimada.

His ordinary life fractures upon receiving a letter from Kotomi, summoning him to an abandoned school building. Arriving there, he finds Rika, homeroom teacher Kumi Takashima, Jinpachi, five female classmates, and Munemitsu Soga—Kotomi's stalker—all drawn by identical letters. The group becomes trapped when the building is sealed by the janitor Isaku, who confines them for unknown reasons.

Trapped inside, Kenta exhibits anti-hero traits, including opportunistic actions like peeking at female characters' underwear. As internal conflicts escalate—fueled by Munemitsu's jealous violence and Takashima's mental decline—Kenta gradually assumes leadership. His pragmatic decisions become vital for survival: searching for keys and exits across the building's floors, with his dialogue choices directly influencing others' evasion of capture.

His relationships critically shape events. His persistent focus on Kotomi and ignorance of Rika's feelings intensifies rivalry with Munemitsu. His bond with Jinpachi shatters upon discovering Jinpachi's collaboration with Isaku. Conversely, his reliability thaws Miyuki's initial coldness, earning her trust.

The outcome hinges entirely on Kenta's choices. Failing to save everyone leads to a bittersweet graduation alongside rescued peers, mourning the lost. Successfully rescuing all results in a conclusion where he enters a romance with either Kotomi or Miyuki. Neglecting certain subplots can trigger Akemi's mental collapse, causing her to set the building ablaze and kill all occupants.