Shuuji Mizuhara lives as a student with his younger sister, Saki Mizuhara, while their parents are away on an extended business trip. Though not biologically related, Shuuji initially maintains a polite and orderly demeanor around Saki. Secretly, he nurtures romantic feelings for her that intensify uncontrollably. These feelings find reciprocation when Saki confesses her own long-held love, leading them into a romantic relationship.

Their new bond is shattered when Saki receives threatening emails from an anonymous stalker. The messages contain compromising photographs capturing her in private moments, like changing clothes. The stalker blackmails Saki, demanding increasingly immoral public acts: self-touching in theaters, using toys in cafes, and exposure on public transit.

Shuuji discovers the stalking after receiving unsolicited nude photos of Saki via SMS, sent following the stalker's first physical assault on her. He notices Saki becoming evasive, avoiding him and giving sporadic answers. Despite recognizing signs of distress—unexplained absences and visible anxiety—his attempts to intervene are met with evasion. Shuuji installs a tracking device on Saki to monitor her location and conversations but fails to involve authorities or confront the stalker directly, even when witnessing public assaults.

As the stalking escalates, Saki grows addicted to the stalker's demands and distances herself from Shuuji without explanation. His role shifts from concerned partner to passive observer, marked by helplessness and suspicion. The narrative concludes without resolution regarding Shuuji's ability to protect Saki from the stalker or the consequences of the blackmail.