The film begins with Amanda (Olivia Cooke) in a stable, tenderly petting her horse, Honeymooner, before she pulls out a knife from her bag. Shortly after, she arrives at the home of her childhood friend Lily (Anya Taylor-Joy) for tutoring. Here, Amanda confesses her emotional struggles, revealing that her inability to feel genuine emotions has pushed her away from Lily, explaining their fractured friendship. To Amanda's surprise, she learns that her mother has been paying Lily to spend time with her, rather than just teaching her.
Eventually, Amanda meets Lily's disrespectful stepfather Mark (Paul Sparks), who constantly engages in rowing exercises, much to Lily's irritation. As they discuss Mark's unkind nature, Amanda encourages Lily to consider an extreme suggestion: to kill him. Lily dismisses the notion, labeling it as foolish.
The narrative takes a darker turn when a party reveals that Amanda has been accused of mutilating her horse, with scandalous images circulating online. Concurrently, Lily encounters Tim (Anton Yelchin), a troubled figure with a criminal past, at the party. Post-party, Lily learns of her own expulsion from school due to a plagiarism incident, and her life only spirals as Mark, her stepfather, displays further hostility towards her mother, Cynthia (Francie Swift).
Tensions mount when Lily and Amanda discuss the increasingly fraught life situations they find themselves in, ultimately agreeing that Mark must be eliminated. Amanda reveals the heavy burden she carries after having killed her horse to end his suffering, thus exacerbating the dire outlook for her future should she be involved in another crime.
Determined, they devise a plan that involves hiring Tim to commit the murder. However, their scheme twists unexpectedly when Amanda discovers damaging evidence of Tim’s past that they can use to blackmail him into following through with their deadly plot. On the appointed day, as Mark remains alone in their home, an unsettling atmosphere builds. Tim, initially hesitant, fails to take action after Mark discovers the hidden weapon.
Desperation leads Lily to concoct a new plot; after spiking her drink to frame Amanda, a chilling turn of events results in bloodshed. The aftermath sees Lily dolling out disingenuous comfort while concealing her own gruesome secret.
In the conclusion, some time later, Lily navigates a college interview and intercepts Tim, who is unaware of the chaos that has ensued. Their exchange unveils the unraveling of Amanda’s fate; she has been institutionalized, while Lily brushes off an inquiry about a letter from Amanda, suggesting her disconnection from the past.
> 'Dedicated to Anton Yelchin.'
