In suburban Connecticut, upper-class high schooler Amanda euthanizes her crippled horse with a knife, resulting in charges of animal cruelty.
Sometime later, Amanda arrives at the home of the more popular and academically inclined Lily. The girls were previously best friends but grew apart after the death of Lily’s father. They meet under the pretense of hanging out and having a casual tutoring session, but Amanda knows that her mother has paid Lily to socialize with Amanda. Lily denies being paid, but Amanda, left emotionless by an unspecified mental disorder, is unfazed. Lily meets with Amanda again, this time voluntarily, and they rekindle their friendship.
Lily lives with her mother, Cynthia, and stepfather, Mark, who she hates. One night, Amanda asks if Lily has ever thought about killing Mark, upsetting Lily. However, tensions flare between Lily and Mark when Mark enrolls Lily in a boarding school for girls with behavioral issues. After seeing him berate Cynthia, Lily reconsiders and calls Amanda about the notion of killing him. She proposes that Amanda perform the murder as Amanda would not experience guilt. However, Amanda believes that her pending animal-cruelty trial would make her an immediate suspect. They decide instead to blackmail drug dealer Tim into murdering Mark while the two girls are out of town. The night of the planned murder, Tim arrives on the property but leaves without killing Mark. The girls agree not to contact Tim again, as his own criminal history will prevent him from alerting the police. Lily impulsively prepares to kill Mark herself but is talked out of it by Amanda.
One night, Lily and Amanda are watching a film at Lily’s home when Lily reveals she spiked Amanda’s drink with Rohypnol so she could stab Mark to death and frame Amanda. Lily attempts to back out of the plan, but Amanda, realizing a life spent without emotions is “meaningless,” willingly finishes the drink. While Amanda is unconscious, Lily murders Mark and rubs Amanda with his blood, crying and holding the unconscious Amanda for comfort.
Some time later, Lily encounters Tim, who now works as a restaurant valet. Following Mark’s death, Lily has once again found academic success and is interviewing for college admission. They talk about the murder (though Lily lies about what really happened), and Lily mentions having received a letter from Amanda, who has been committed to a psychiatric hospital for the crime. The letter is shown to detail Amanda’s life at the hospital, including a recurring dream about a future in which humans let the world fall into disarray due to their vanity, leading to it being overrun by thoroughbred horses. When Tim asks what the letter said, Lily says she threw the letter away without reading it.