Chisato Koizumi attends a prestigious girls' school with her friend Tomomi Fujihira. Alarm bells ring when their geography teacher, Mr. Naganuma, summons Chisato alone to the preparation room. Despite explicit warnings from Yasuko Yoshimura about potential danger, Chisato ignores the risks and keeps Tomomi in the dark. She meets Naganuma solo, unaware his motives are predatory.

Afterward, Chisato shows unsettling changes in behavior, catching Tomomi's sharp eye. Tomomi confronts Naganuma about her friend's transformation, only to walk straight into a trap he sets. She evades his scheme and secures vital evidence against him. Cornered, Naganuma then forges an alliance with Hanyuu—the man embroiled in an adulterous affair with Tomomi's mother, Junko Fujihira—to jointly target the Fujihira family.