Twenty-year-old student Kou Ichinomiya has always followed his elite family's motto: never be indebted to anyone. However, an unfortunate situation puts his life in danger, and he is saved by a stranger named Nino, who claims to be from Venus. As if owing someone is not bad enough, the peculiar girl wishes that he repay her deed by experiencing love together—and he must live with her under the Arakawa River bridge.

Although Kou figures that he will somehow fulfill this obligation, his plans are blown away by Nino's friends: various riverside residents who seem even more eccentric than his new lover. From the Village Chief who gives Kou a new name, "Recruit," to a gentle-looking but sadistic woman managing an animal farm, one can never tell which outlandish character will next color Arakawa's bizarre life under the bridge.