While on vacation, Fay's father, played by Rhys Ifans, receives a shocking diagnosis of multiple sclerosis. This life-altering news prompts Fay, portrayed by Sophie Turner, to return to school, where she is excitedly cast in the school production of Macbeth as Lady Macbeth, sharing the stage with Drew, a boy she has feelings for, played by Gregg Sulkin. However, during her first rehearsal, nerves get the best of her, leading to forgotten lines and increased anxiety. To help, her teacher, John, played by Jonathan Rhys Meyers, recommends she watch Roman Polanski's 1971 adaptation of Macbeth for inspiration.
As Fay heads to the school library to find a copy, she is struck with fear when the lights begin to flicker ominously, and she senses someone chasing her. Just as panic sets in, she escapes and meets her friends, who were oblivious to the commotion. In the days that follow, Fay increasingly notices a girl who bears an uncanny resemblance to her, casually passing her by on the street. The situation grows more complicated when Fay witnesses her mother seemingly leaving with another man – someone who isn't her father. Adding to her confusion, a neighbor tells Fay she saw her walking down the stairs of their building, but Fay vehemently denies it was her, even as the neighbor insists she couldn't mistake her due to her distinctive long red hair.
Desperate and fearful, Fay finds herself chasing after the mysterious girl but to no avail. Returning home, she decides to cut her hair into a jagged bob, partly in a symbol of rebellion against her newfound paranoia. When she shares with her friends her concerns about being stalked by a doppelgänger, they suggest it might be Monica, her rival and understudy, who shares the same fiery locks. After revealing her fears to her mother, Fay learns a shocking family secret: she was a twin, but her sister, Layla, passed away during a late-term miscarriage.
After a successful rehearsal ends with a date with Drew, she is jolted back to reality when she finds her father collapsed on the floor. In the midst of a chaotic moment, Fay discovers he had been looking at a sonogram depicting her and Layla. School becomes a battleground of doubts when Fay notices John's car matches that of the man she previously saw her mother with, leading to further confusion and belief that her doppelgänger is involved.
During a frenzied encounter in the school courtyard, Fay confronts Monica, who has cut her hair similarly to Fay's own newly styled bob, leading to an all-out attack fueled by her paranoia. Subsequently, when she confronts the principal about her accusations against Monica, she learns that there’s no possible way that it could have been her in the schoolyard, causing Fay to spiral deeper into her madness. In a desperate act for recognition, Fay creates a scar on her hand to differentiate herself from her double.
Tension escalates when Fay talks with her mother about the affairs she believes are tearing their family apart. Drew's visit goes well, leading to a momentary sense of happiness as Fay feels the pull of her double fade. But after her father's health takes a turn for the worse, her mother’s relationship with John rekindles, causing Fay's paranoia to resurface. One day, while walking with Drew, she finds John's car crashed into a tree, and the police inform her of a girl who caused the accident, prompting her belief that her twin was responsible.
Driven by despair, Fay skips school to visit John in the hospital, while Drew expresses gratitude for a private picture he found, further complicating her emotions. During rehearsals, the principal instructs Fay to recite her lines exactly as she did before, but Fay insists that she wasn’t present, further isolating her.
In a poignant conversation with her father, he reveals that he can see Layla too but warns her against looking directly at her sister, as it could cost Fay her life. An emotionally charged moment leads Fay to the local underpass where she reaches out to Layla, pleading for her sister to leave the family alone. In a chilling turn of events, instead of yielding to Fay’s requests, Layla commands her to look upon her, which Fay reluctantly does.
Tragedy strikes as she returns home only to be met with the devastating news that her father died while attempting to keep her from confronting Layla. In the aftermath, Fay and her mother embrace, finding solace in shared grief.
Time passes and the night of the Macbeth opening arrives. Backstage, her mother comforts her, assuring her that her father would be proud of her performance. As Fay gazes at her reflection, she removes the bandage from her hand, horrified to discover she lacks the scar she so deliberately cut into herself; instead, the visage staring back at her is that of Layla. As a sinister smile crosses her sister's face in the mirror, Fay steps forward to perform, leaving behind the image of herself trapped, forever a prisoner of her own making.
