Two upper-class teenage girls in suburban Connecticut rekindle their unlikely friendship after years of growing apart. Together, they hatch a plan to solve both of their problems-no matter what the cost.
- The movie opens with a young woman, Amanda (Olivia Cooke), in a stable with her horse, Honeymooner. She gently pets him and then reaches into her bag and pulls out a knife.
Amanda is brought to the home of her childhood friend Lily (Anya Taylor-Joy) so that she can tutor Amanda. Amanda tells Lily she can't seem to feel genuine emotions, and her behavior is frequently off-putting to Lily, which is why they haven't been friends in so long. Amanda also finds out that her mom paid Lily to hang out with her instead of just tutoring her, although Lily tries to deny it.
Amanda meets Lily's stepfather Mark (Paul Sparks), who is rude to Lily and annoys her by constantly using a rowing machine. Noticing that Lily hates Mark, Amanda prods her with questions until she eventually suggests that she should kill him. Lily thinks it's a dumb plan.
Some time later, Lily attends a party where her peers are talking about how Amanda mutilated her horse and that there are pictures of the crime scene floating around the internet. Outside, an argument is taking place between the party's host and an older guy named Tim (Anton Yelchin), who is trying to hit on the guy's sister. Another guy notes that Tim did jail time for statutory rape. The first guy hits Tim in the face and makes him leave. As Lily is leaving the party, she runs into Tim, who tries to engage her in conversation, but she is not interested.
Lily finds out from her mom (Francie Smith) that she got expelled from her private school for plagiarizing a paper, and now she is being sent to a different school, which Mark has already paid for. Lily responds by going to sabotage Mark's rowing machine, which later results in him having a broken nose and wrist. When Lily's mom tries to help him, Mark acts like a dick to her. This brings Lily to meet with Amanda again to decide that she definitely does want to kill Mark. However, Amanda notes that it wouldn't be easy since she is already awaiting trial for animal cruelty, so if she were anywhere near the crime scene, she would be screwed. Amanda reveals that she killed Honeymooner because he was in pain and couldn't walk anymore. She tells Lily they need to establish alibis and be away from the area.
The girls find Tim working at a retirement home and they hire him to carry out the hit on Mark. He goes with them to Lily's house where they plan to set things up for the murder, and that it needs to be the upcoming Saturday since Lily is going to a spa with her mom and Amanda takes psychotherapy sessions. When Tim sounds hesitant, Amanda plays an audio recording of Tim selling hard drugs to a minor. Tim pulls out a gun and taunts the girls by pointing it at them until Amanda knocks him over the head with a lamp, resulting in a head injury. The girls blackmail Tim into going through with the plan or they will send the audio to the police.
Come Saturday, Mark is home alone. He notices the light from outside go on as Tim is creeping through the back, but he only goes to get his gun that the girls hid in a grill, and he leaves Mark alone.
The girls realize Tim failed to carry out the hit, but they know he can't report them due to his own criminal record. Lily and Mark later argue over him sending her to a boarding school while Amanda hides while holding a knife. Mark chastises Lily for smoking in the house, to which Lily responds that he should leave her mother if he really cares about her. Mark tells her he is going to cut off her funding after the school year is up.
At night, the girls are watching a movie when Lily admits to Amanda that she spiked her screwdriver with Rohypnol so that she can kill Mark herself and frame Amanda for it. Lily tries to back out, but Amanda chugs the drink and tells her to go through with the plan because she feels her own life is going nowhere. After Amanda passes out, Lily goes upstairs. We hear thumping sounds, and Lily runs back downstairs covered in Mark's blood. She rubs some of it on Amanda and lays next to her.
A while later, Lily arrives at a restaurant for a college interview. She finds Tim working there as a valet. He heard about Mark's murder, but Lily is clearly doing fine. Tim asks about Amanda, who was sent to a mental institution after being framed. Lily tells Tim that Amanda sent her a letter. We hear Amanda's voice as we see her in the institution, with her describing a dream to Lily about Amanda being a horse, and another where she is Honeymooner and her spirit rises to see time speed up where new people are moving into the neighborhood and making things better. When Tim asks Lily what the letter said, she says she just threw it out.
'Dedicated to Anton Yelchin.'