From the Animeraider:

This is the tale of two distant cousins, Rio and Ryoutarou – who met as 4 year-olds at the wedding that joined their two families. They were the smash hit of the wedding (even kissing and saying they would get married too – hey, 4 year-olds are cute that way) but they didn’t see each other again after that.

It’s now ten years later and Rio is the leading scorer on the women’s basketball team, dating Yasuhiko, one of the top scorers on the men’s team. She goes to a game and discovers Ryoutarou playing for the opposing team – and he’s their leader. They’re reunited again and both seeing other people, but with that now decade old promise will their paths converge again? The storyline covers the following 5 years, so anything is possible!