The story revolves around Kouhei Orihara, the main protagonist. His father died when he was very young, followed by his younger sister Misao and finally his mother. Deeply depressed, he goes to live with his aunt Yukiko, his mother's younger sister, about ten years before One's story begins on November 30, 1998. After he meets Mizuka Nagamori as a child, Kouhei begins to seclude himself in the Eternal World, a world within his mind.
(from Wikipedia)