Ichinomiya Kou has always lived according to the creed of his wealthy, successful family: never be in debt to anyone. But one day, after having his pants stolen and hung on the trusses of the Arakawa Bridge by a pack of rogue kids, his life is saved by a homeless girl named Nino. In order to pay her back, Kou promises to be her boyfriend; and thus begins his new life under the bridge.