Teenager Ellie and her parents are moving house, but on the way to their new home are involved in a near-fatal car crash. Reeling from the shock of losing one of her parents, Ellie starts her new high school, and a mysterious girl, Madeline, enters into her life. Madeline is everything that Ellie isn't: beautiful, bold and brave. Exactly what she's always wanted in a best friend - and wished for herself. But as Madeline's influence on Ellie grows, and her life begins to spiral out of control, she starts to question what's real. Is Madeline really who she says she is? Can Ellie trust her - and, more to the point, can Ellie even trust herself?