Natsuo is a high school boy who is experiencing the crushing despair of unrequited love. To make matters worse, the person he is in love with is his teacher, Hina. In an attempt to lift his spirits, he attends a mixer where he meets a girl named Rui. The two sleep together, expecting never to see one another again, but fate has other plans. His life suddenly becomes more complicated when his father comes home and announces he has remarried a woman with two daughters whom Natsuo has met before: Hina and Rui!

Following a failed confession to her first love, female high school student Mikoto Ochiai contemplates jumping off the roof of the school building. However, her thoughts are interrupted by the arrival of Jin Haiba, the physics teacher known for being a slacker and the butt of students' jokes. Calmly, he lights a cigarette and asks for an explanation of the events that led her to this precipice. Rather than dissuade her from killing herself, he makes a request—will she enter into a romance with him before she dies? Together, the two gradually ponder why they should keep living and why they should not love each other in earnest.