Taken from Stephen Paul's website:

Sayuri is a story set in a college club, or "circle". A young man and woman are the leaders and organizers of the circle and have known each other since childhood. Naturally, the boy (Naoya) is much too close to the girl (Tomoko) to see her as a romantic interest, but she does love him. [So far pretty unremarkable, right?] The story begins when an attractive new underclassman, Yuki, joins the circle and immediately grabs the attention of all the male members. As a matter of fact, her face is identical to "Sayuri," the imaginary girl that Naoya has used for masturbation material ever since he was old enough to jerk off.