Ikeda Masaki, called Ikeuchi by his friend, is a college student living alone with his little sister, Minami, who is in grade school, after their father suddenly left one day leaving only a note behind. After thinking about dropping out of university to work and support himself and his little sister, Ikeuchi was encouraged by his best friend, Kanda, who currently has a job after dropping out himself, to continue his studies, and if ever the need arises, Kanda would be there. Ikeuchi believes that even if misfortunes come, as long as he has friends and family, they will be happy.
As time goes on, Ikeuchi gets any part-time job he can get to pay the bills. He eventually forgets Minami's birthday, but is saved by Kanda, who not only remembered it, but bought a gift that the little girl really liked. Ikeuchi notices strange behavior on Kanda's part sometimes, however, he pays it no mind.
One day, Ikeda gets a job that requires him to leave home for a while, so he asks Kanda to stay with Minami. Kanda intially furiously refuses, telling Ikeda not to depend on him all the time. When Ikeuchi is about to leave saddened, Kanda reluctantly changes his mind. After he finished his work at a hotel, Ikeuchi returns earlier than expected because he believes that his little sister is missing him, as she called him to inquire about the time that he will be coming back. Since it is late at night, he does not phone her when he gets off the train, but heads straight home instead.
Ikeda goes to Minami's room as soon as he walks in, and knocks on the door. She is surprised that it is her brother, and when he opens the door, Minami screams hard for him not to enter . . . What Ikeda sees there shatters everything he believes in . . .