Yaleen, the river-woman, has always been involved with "the black current," a strip of black running along the center of the river that separates the eastern bank from the mysterious western side. No one is really sure what the black current is, but the people sense that it is alive and powerful, as it allows only women into the river and brings madness and death to men who enter more than once. Longing to experience that freedom, Yaleen drinks of the black current and joins the River Guild. But when her brother Capsi discovers a way to cross to the forbidden western side, Yaleen is caught in the middle of a battle that could end the world.