Miku Haneoka attends Shirakawa Academy, possessing a mentally slower disposition yet demonstrating notable talent in rhythmic gymnastics. Her inherently friendly and trusting nature sets her apart from peers who express revulsion toward the overweight gym teacher Nikusuke; she interacts with him without discomfort. This openness leaves her vulnerable to manipulation by Tomomi Shirakawa, the board chairman's granddaughter. Tomomi exploits Miku's desire for friendship to recruit her as the inaugural member of a clandestine nighttime club dedicated to "Dark Rhythmic Gymnastics." Under Tomomi's orchestration and Nikusuke's direct instruction, Miku endures intense training involving coercive and painful activities, including rape and torture, resulting in significant psychological and physical trauma.

Later depictions show Miku's height increasing from 147 cm to 152 cm, with lingering effects from the training regimen persisting. Her younger brother, Fubuki, grows concerned about her deteriorating condition and eventually locates her. As they are not biologically related, Fubuki employs explicit hentai-inspired techniques to "aid" her recovery. Associated video game content depicts Miku pregnant, her weight rising to 49 kg and measurements changing to 81-88-82 cm.

Following her initial victimization, Miku undergoes a shift in allegiance, transforming from a target of Nikusuke's abuse into a compliant participant exhibiting newfound loyalty to him. This change manifests when she collaborates with other traumatized girls to facilitate Tomomi's coerced initiation into the club, actively partaking in the retaliation against her former manipulator.