Emi Hayama, a high-school student, cohabits with her younger step-brother Tomomi. She cultivates profound romantic feelings toward him, intending to confess on their mutual eighteenth birthday. Before the confession occurs, debt collectors seize her late father’s assets over unpaid obligations, triggering Tomomi’s abrupt vanishing. This compels her quest to locate him and declare her love.

Her search precipitates psychological fragmentation, birthing a separate identity—Amy. Emi’s core self remains timid, reserved, and easily destabilized by adversity, historically leaning on Tomomi for support. Conversely, Amy personifies Emi’s suppressed sexual yearnings, radiating assertiveness, control, and an obsessive drive to enact erotic fantasies, primarily involving Tomomi or strangers met along her path. This alter ego increasingly materializes, blurring Emi’s grasp on reality versus elaborate daydreams.

Emi routinely disavows the Amy persona, demanding others refrain from using the name. Yet she grapples with reconciling memories, sometimes suppressing distressing truths—such as observing Tomomi with other women—by deeming them implausible fabrications. Her fantasies frequently depict graphic encounters with Tomomi, intensified by a biological revelation: his rare blood type releases pheromones at eighteen, provoking compulsive female attraction. This twist amplifies turmoil in her pursuit.

Her odyssey immerses her in sexual episodes engineered by Amy—including bondage, group dynamics, and same-sex liaisons—sparking inner strife between her inherent modesty and Amy’s unrestrained impulses. The quest culminates in collapse when Tomomi leaves after their reunion, hinting at a potential fusion or lingering discord between her fractured selves. The narrative closes without clarifying her mental reconciliation or future with Tomomi.