Natsuo Fujii harbors affection for his teacher, Hina. In an effort to move on, he attends a social gathering and crosses paths with Rui Tachibana, a peculiar girl. Rui surprises Natsuo by inviting him to her home with a unique request—for him to engage in a physical encounter devoid of emotions, merely for educational purposes. Hoping it might help him let go of his feelings for Hina, Natsuo reluctantly agrees. But this singular event leads to a new dilemma for Natsuo as his father's remarriage introduces him to his new stepsisters, who happen to be none other than Hina and Rui. Caught in a complex love triangle under one roof, Natsuo navigates the intricacies of this unexpected situation on the brink of maturity.