As the story opens, Sage Priestly, 17, is running for class president against Mona, whose popularity Sage finds both threatening, fascinating, and a matter that keeps her in a state of uncomfortable envy. In her efforts to "be Mona," Sage undertakes a drastic diet, changes her haircolor, and focuses all her leisure dream time on Roger--a boy she can't see is incipiently abusive, though her long-time friend, Vern, loves her in a healthy and faithful way--a love that is tested when Sage starts dating Roger and suffering actual physical abuse.  As we learn about her troubled social life, we also learn that at home Sage is a caregiver for her single mother whose bipolar disorder  and depression pose a huge and confusing challenge to the teenage daughter.  Vern's parents eventually intervene to help both Sage and her mother get appropriate care and oversight, and Sage begins to recognize in Vern (and his gay friend Walter, who has suffered his own social challenges) the kind of friend that will last.  The book includes an afterword in which the author provides a note from personal experience on bipolar disorder (one of her parents was bipolar) and abuse, and lists helpful resources.