High schooler Natsuo is in love with his teacher, Hina. Trying to get past his feelings, Natsuo goes on a group date with his friends and meets a quiet girl named Rui Tachibana. Uninterested in the event, the two sneak out. She takes Natsuo to her house and then asks him to have sex with her, although she is not romantically interested in him. Afterwards, his widower father tells Natsuo that he is going to remarry, and Natsuo finds that his new stepsisters are Hina and Rui. Following the marriage, they move into a new house together. As everyone is going to bed, Natsuo tries to kiss a sleeping Hina but is interrupted by Rui.