When a sociopathic teenager named Amanda (Olivia Cooke
), who comes from a wealthy Connecticut family, loses all her friends after she kills her horse, her mother hires Amanda's ex-best friend Lily (Anya Taylor-Joy
) to hang out with her.
For her part, the prim and proper Lily hates her wealthy stepfather Mark (Paul Sparks), who constantly berates her mother and is disgusted with Lily for getting kicked out of her private school for plagiarism. When he decides to send Lily to school for teenagers with severe behavioral problems, the two girls decide to get rid of him.
They blackmail Tim (Anton Yelchin), a lowly drug dealer, into committing the murder for them.