One day, due to a parent's remarriage, an older brother suddenly gains a cute younger stepsister named Yuzuno, and he's troubled about how to interact with her. Amidst this, Yuzuno comes to him saying she has something to discuss. When he hears her out, a shocking truth is revealed: she has been having a sexual relationship with her father! The older brother's feelings of wanting to protect Yuzuno and his envy become intertwined. In the end, the older brother and Yuzuno also develop a physical relationship.