The narrative centers on Mitsuki Kanzaki, a high school girl who finds her life thrown into a bizarre and chaotic spiral after her father remarries, making her the step-sister to Yuya Kanzaki, a boy her age. Initially, the two teenagers experience the typical awkward tension of becoming sudden siblings under the same roof, struggling to find common ground. However, this domestic friction takes a supernatural and wildly absurd turn when Mitsuki is suddenly possessed by the ghost of a hyperactive, mischievous little girl named Hiyori.Hiyori’s spirit is bound to the earthly plane by a peculiar and highly specific regret: she died before she could fulfill her ultimate
romantic fantasy of experiencing youth as a devoted, doting younger sister to an older brother. Because of this lingering obsession, Hiyori takes over Mitsuki’s body and immediately designates Yuya as her target brother. To resolve the haunting and release the spirit so Mitsuki can reclaim her normal life, Yuya discovers he must actively participate in Hiyori’s elaborate, dramatic, and frequently embarrassing demands. These tasks are designed to simulate classic sibling tropes, but they rapidly escalate into surreal, passionate, and boundary-pushing scenarios that blur the lines between sisterly affection and overt romance.As Yuya reluctantly plays along to help his step-sister, the dynamic between the trio grows increasingly complex. Hiyori’s ghost pushes Yuya and Mitsuki into compromising situations, forcing them to confront hidden emotional vulnerabilities and undeniable physical attractions. Mitsuki’s consciousness is often trapped inside her own mind, helplessly watching her body act on impulses that she is too proud to admit she shares. Meanwhile, Yuya navigates the fine line between acting as a supportive brother to appease a ghost and dealing with his very real, deeply confused feelings for his actual step-sister.Throughout the escalating chaos, the storyline balances comedic embarrassment with genuine
emotional growth. Yuya’s unwavering dedication to solving the haunting forces both teenagers to communicate more openly than they ever had before. As they weather Hiyori’s relentless tests of devotion, the barrier of sibling awkwardness completely dissolves. Ultimately, the supernatural interference acts as a catalyst, breaking down their emotional defenses and paving the way for a deeply intimate connection. By the time Hiyori’s spirit finally finds peace and prepares to move on, Mitsuki and Yuya are left alone, transformed by their bizarre ordeal and ready to embrace a profoundly passionate, romantic relationship that transcends their nominal step-sibling bond.