Despite his strong attraction to his teacher Hina Tachibana, Natsuo Fuji knows that pursuing a relationship with her is impossible. However, persuaded by his friends, he attends a party and meets Rui. After an unexpected turn of events, both lose their virginity. Though it isn't what they envisioned, they both move on without seeing each other again. That is until Natsuo's father informs him that he is remarrying, and Natsuo will gain two new step-sisters - Hina and Rui. This creates a problem, as Natsuo is attracted to Hina and has a shared history with Rui.