Koshiro and Nanoka, two young people both recovering from heartbreak, meet and develop a mutual attraction towards each other... until Koshiro's father informs him Nanoka is his daughter, and thus Koshiro's sister.

Koshiro struggles with his feelings for Nanoka, when she comes to his office he snaps and yells at her, enraging her and making her go out with her friends away from home.

Koshiro continues to battle with his feelings for Nanoka.

Koshiro bumps into his ex-girlfriend Shoko, but she has moved on with a new man. A frustrated Koshiro finally faces the fact that he is attracted to Nanoka.

Koshiro gets drunk and again yells at Nanoka. The next day, her birthday, he tries to make amends for his misdeed.

Nanoka comes home from school with a male classmate, which rouses Koshiro's jealousy.

Nanoka receives a love letter at school and consults Koshiro, who is indifferent about the matter at first but is later wracked with envious regret...

Koshiro looks back on his early years, when he and Nanoka were innocent children and their parents were still married. He resolves to be the best brother for Nanoka.

Nanoka comes to Koshiro's office, and his workmate Kaname says they make a nice couple. Later, Nanoka declares her love to Koshiro, but he angrily brushes her off.

An agonized Koshiro decides to move out of his house, with Kaname's support. He tells Nanoka not to meet him again, to each other's heartbreak.

Kaname figures out the true nature of Koshiro and Nanoka's relationship, and confronts Koshiro about his actions.

Kaname discovers Nanoka at Koshiro's house, but neither Nanoka nor Koshiro can deny their love for each other.

Unable to live with or without each other, Koshiro and Nanoka make a momentous decision: should they live, or die?