Juliet Juliet and Vivian Lowe are twin sisters in their final year at Lindberg Academy, a prestigious boarding school for the performing arts. Both are classical pianists; Vivian, a prodigy, has already been accepted to Juilliard, while Juliet, overshadowed by her sister, has decided to take a gap year.
The violinist who died by suicide is Moira, a senior who was scheduled to perform at the senior showcase. Auditions are organized for her replacement. Both sisters sign up, with Juliet deciding at the last minute to play the same piece as Vivian—the Saint-Saëns Piano Concerto No. 2.
After finishing practice, Juliet follows the sound of the violin to Moira's room and discovers a sun symbol on the wall that matches one in Moira's theory notebook. She plays an untitled piece from the notebook, but Dr. Cask interrupts and reveals the name of the piece, The Devil's Trill by Giuseppe Tartini, and confronts her about changing her audition without telling Vivian.
During the audition, Juliet enters a dreamscape and sees herself at the showcase receiving roses. She wakes to find she had fainted during the performance. She discovers a drawing of three balls in her hand, corresponding to tablets she took. Her relationship with Vivian grows strained as she learns that Vivian has landed the showcase.
Juliet argues with her mentor, Roger, accusing him of underestimating her. The confrontation leads to Roger’s suspension after he slaps her. Juliet then asks to be reassigned to the prestigious Dr. Cask, who is also mentoring her sister, and later finds bloody tampons on her shelf. She confronts Vivian, who simply tells her they’re now “even.” When Dr. Cask asks why she changed her piece, Juliet admits she only wanted to beat Vivian, and Dr. Cask tells Juliet she is Vivian's equal in technique but lacks her passion.
Juliet is invited to the secret senior party, where she overhears Vivian on a call telling someone she is “done fucking” them. Juliet flirts with Vivian’s boyfriend, Max, and tells him about Moira's notebook. Vivian catches them and the two fight, causing Juliet to run off. A blinding light—similar to the one seen in Moira's notebook—appears and halts Juliet. Vivian, unable to see it, chases after her and falls off a cliff. The next day, Juliet wakes at her desk with several pages bearing Vivian’s name repeated, and she notices the third page matches Vivian’s fall, while the second aligns with Roger. Juliet learns Vivian injured her arm in two places, risking her career; Juliet is offered Vivian’s role and accepts, while Vivian is suspended. When Juliet tries to talk to Vivian, she’s accused of sabotaging her sister. Looking up Moira’s suicide online, Juliet learns that her mother fell from a ski-lift into a coma and her father burned down their house and himself.
Juliet confides in Max about Moira and the notebook, reveals that Vivian cheated on him, and they break up. She then sleeps with him. On her birthday, Juliet invites Dr. Cask to a dinner with Vivian. Vivian erupts, throwing the cake onto him and storming off. As her father is drunk, Juliet is driven home by Dr. Cask. He tries to offer comfort, but when he rejects her kiss, she reveals she discovered he was having an affair with Vivian and burns his trophy. Juliet realizes that the preceding events align with drawings in Moira’s notebook. With the last page torn out, she enters a trance and, through automatic writing, produces an illustration demanding sacrifice, then panics and burns the entire notebook.
Before the showcase, Vivian confronts Juliet and declares she will always be mediocre. Juliet suffers a panic attack on stage and runs to the roof of the auditorium. She imagines a version of herself finishing the performance to a standing ovation and the approval of her sister. In the end, Juliet’s body is found on a statue, bloody, as campus students go about their day, unaware of the tragedy that unfolded.
