Following Katy's recovery in What Katy Did, Katy and Clover go to Hillsover, a boarding school. Together with their new friend Rosamond Redding, or "Rose Red", they form a Society for the Suppression of Unladylike Conduct--but then they are accused of sending notes to a boy outside the school. Katy and Clover know they have nothing to do with it, but can they prove their innocence to the teachers and principal before they are expelled?