Sayaka Matsumoto is a nineteen-year-old student at an elite all-girls boarding school, distinguished by her chest-length brown hair and striking green eyes. She maintains a clandestine romantic relationship with her roommate, Atsuko Yoshikawa—a bond forged since high school and hidden from Atsuko's older sister. Within their shared dorm, their intimacy unfolds through passionate embraces, fervent kissing, and sexual encounters, though these moments face interruptions like knocks from peers inviting them to dinner.

Sayaka exhibits fierce possessiveness and jealousy toward Atsuko, particularly when Atsuko's athletic excellence in lacrosse draws admiration from other students. She craves Atsuko's undivided attention and resents her interactions with others.

The school environment is plagued by supernatural disturbances, including eerie creature sightings and institutional corruption. Sayaka drifts through routines with listless disinterest, a malaise temporarily eased only by Yuko Yoshikawa's violin music. She participates in campus rituals like semester-opening ceremonies and communal meals.

Her story revolves around shielding her secret romance while confronting the school's unsettling phenomena.