姉を助けるため残された姉妹を穢す背徳の日々

Ogura Shinichi lost his mother as a child and gained a new family through his father's remarriage to a stepmother and her three daughters. Despite his father and stepmother being frequently absent due to work, they live a peaceful life together. However, their tranquility is shattered when the oldest daughter, Kasumi, is kidnapped. The kidnapper demands that Shinichi violate one of his younger sisters in exchange for Kasumi's safety. Initially doubtful, Shinichi is shown a distressing video of Kasumi that takes a toll on his mental state. Succumbing to the kidnapper's influence, he violates his second sister, Satomi, and even third sister, Yuu. Sadly, Kasumi remains captive, and the situation worsens as Shinichi's childhood friend, Makoto, and his teacher, Naoko, become involved. Isolated from their parents and the authorities, Shinichi and his sisters experience a gradual erosion of their sanity. How long will this abnormal and never-ending nightmare continue?