A young upper-class elite man, Ichinomiya "Riku" Kou, who has a policy of not ever wanting to be in debt to anyone, ends up owing his life to a strange girl, who rescued him. His saviour is Nino, a beautiful homeless girl, who believes that she is from the planet Venus and lives under the bridge by the Arakawa river. So the young man ends up living there too.

[Source: AniDB]