Sarah Weaver, a jaded seventeen-year-old from a broken family, leaves California to attend an all-women's college in Massachusetts. At Wetherly, Sarah meets Maddy Snow and Agnes Pierce, a mysterious pair of legacy students who have been best friends since birth. When the girls accept her into their duo, loner Sarah finally has the family she's always wanted.

But then she starts to notice some strange and disturbing things: Maddy's compulsive lying, Agnes's obsession with Maddy, and the deterioration of the girls' friendship. And just when Sarah begins to question her own sanity, shocking secrets come to light that will bring their friendship to a new level of destruction.