Atsuhiko is a university student majoring in marine biology. He doesn't go to university much and stays in a simple room that was converted from a storeroom in his parents' house, leisurely continuing his research. There is a presence that threatens Atsuhiko's peaceful daily life. That is his younger sister, Miu. When she was little, she was a cute little sister who followed Atsuhiko around, but at some point, her breasts started to grow and she grew into a woman's body. Atsuhiko tries to distance himself from his sister, but behind the scenes, he somehow manages to stay calm by imagining his sister's lewd acts every night and venting his frustrations. This morning, his sister, unaware of Atsuhiko's efforts, is standing next to him, brushing her teeth in light clothing. Meanwhile, it seems that his sister has finally found a boyfriend...