Chiaki Honda is a middle-school boy who neglects his studies. Furthermore, he smokes, and has dyed his hair a bright green. One evening at the arcade, a girl challenges him to a match. The girl, who straightforwardly introduces herself as Nanako Kikuchi, suggests a bet, nonchalantly wagering her body for Chiaki's watch. Chiaki is soundly defeated, and he goes home fuming.

His life goes on, of course. The next day, he meets a strange young man on the train who looks an awful lot like him. And eventually he runs into Nanako again.

(Source: MU)