Following their parents' divorce, Akira takes on a paternal role to care for his younger sister, Yui, but his protective instincts soon devolve into intense, carnal desire. Driven by jealousy and repressed urges, Akira begins acting on his attractions while Yui is sleeping, unaware that she is often secretly aware of his nighttime advances. This series explores the blurred lines between familial care and forbidden desire as the siblings' relationship shifts into a complicated sexual dynamic.