Hanakyuu

The main character is Toshiaki Uemura, a typical university student living alone in the big city. The story begins in Christmas night, when Toshiaki and his girlfriend Sumomo fulfill physically their love for the first time. Their future seems now firmly set, but the next morning, while Toshiaki and Sumomo are on the way to their university (in Japan, Christmas is not a holiday), an unknown woman, Mayumi, approaches Toshiaki with an unusual request: "Do you want to care for this flower seed?"

[Edited from a forum post by Baldo]