Hayasaka Akihito is one of the few male students at the newly co-ed Saint Francesca Academy, where he balances his school life with secret romantic encounters with his childhood friend Yuika and his senior, Kisaya. When his younger sister, Umi, begins acting cold and avoiding him, Akihito seeks the truth behind her behavior. Through a conversation with Yuika, Akihito recovers a suppressed memory of a tragic childhood accident involving Umi, revealing the emotional burden his sister has been carrying in silence.
Akihito navigates the romantic tensions of his classmates, including Riru's concerns over Reika's distant behavior, while grappling with his evolving feelings for his non-blood-related sister, Umi. After realizing that the distance between them stems from mutual attraction, Akihito decides to be honest about his emotions. He eventually confesses his love to Umi, leading the two to transition from siblings to lovers.