Natsuo Fujii, a teenager, grapples with unrequited love for his cheerful teacher, Hina. Seeking solace, he attends a mixer where he meets Rui, a girl equally disinterested in excitement. Their bond leads to an unexpected proposal for sex, driven by Rui's desire to break free from childish perceptions. Despite their shared dissatisfaction, they part ways. Soon after, Natsuo's world is upended by his father's sudden remarriage to Tsukiko Tachibana, introducing him to his new step-sisters—Hina and Rui. As complex emotions unravel, Natsuo navigates the intricacies of forbidden love in his unconventional family dynamic.