Asuka Momoki, an employee at a major advertising company, has a secret. He's 22 years old, impotent, and still a virgin. The existence of Asuka's younger step-brother Yui has a lot to do with it.

Asuka and Yui were the best of friends until an unfortunate incident one summer's day. Ever since then, Yui has avoided Asuka at all costs, even going so far as to leave Japan and study in America.

A successful tennis player, Yui no longer needs a big brother, so where does that leave poor Asuka? What happens when brotherly love transforms into something more?