Middle-class Rosie Macalister has worked for years to fit in with her wealthy friends on the Yale equestrian team. But when she comes back from her junior year abroad, she finds that the group has been infiltrated by a mysterious intruder. Annelise Tattinger is unlike anyone Rosie has ever met. But when one of their friends notices money disappearing from her bank account, Annelise's place in the circle is thrown into question. It's only after graduation that Rosie uncovers Annelise's true identity - and how her place in their elite Yale set was no accident. Is it too late for Rosie to put right what went wrong, or does everyone's luck run out at some point?