The film begins with David Strine (Joshua Leonard) expressing his fondness for a woman in blue, recalling their first meeting when she wore that color. The story centers on Sawyer Valentini (Claire Foy), who works as a sales representative in Pennsylvania. One day, she believes she spots David at her workplace, sending her into a state of fear. Living alone, her only friend at work is Jill (Sarah Stiles), while she frequently communicates with her mother, Angela (Amy Irving), who resides in Boston.
After a tense Tinder date, where Sawyer bluntly informs her date that he will never hear from her again, her panic attacks resurface, leading her to lock herself in the bathroom. Seeking help, she visits a counselor at Highland Creek to unpack her tumultuous experiences. She shares that she endured two years of stalking from David, which has taken a significant toll on her mental health, even leading her to contemplate suicide. The counselor suggests she sign up for further counseling to effectively process her feelings.
Once Sawyer completes the necessary paperwork, she finds herself involuntarily committed to the facility for an extended period. During her stay, she meets a diverse group of patients, including Nate Hoffman (Jay Pharoah) and Violet (Juno Temple), the latter of whom poses a threat by brandishing a shiv and threatening to cut Sawyer's hair. A disturbing encounter occurs when another patient, Jacob, attempts to assault Sawyer, prompting her to defend herself violently.
The following morning, she sees Dr. Hawthorne (Gibson Frazier), who informs her that her violent actions will result in a week-long confinement. To her dismay, she discovers that one of the orderlies is, in fact, David, now under the alias "George Shaw." Trying to convince others of the truth leads to her further restraint and isolation.
In the midst of her struggles, Sawyer learns from Nate that Highland Creek’s operations are essentially an insurance scam, exploiting patients for profit under the guise of treatment. As a bond forms between Sawyer and Nate, she recounts the traumatic history with David—how he became obsessed after her work with his father in hospice care, escalating from texts to terrifying break-ins. Upon filing a restraining order, authorities gave her advice on evading him, yet his fixation didn't wane.
The tension culminates when David assaults Nate, leaving Sawyer in a state of panic. Yet, amidst her violent outbursts, she is often confined and isolated, leading to another encounter with David. He appears to be grasping for a connection, suggesting an escape to a cabin, which Sawyer dismisses harshly, shaking his delusions of love. Meanwhile, disturbing secrets surface, including a horrific discovery of a body linked to the ongoing chaos.
As Sawyer fights for her freedom and safety, faced with David's violent retaliation and a harrowing game of life and death, she retrieves a weapon and confronts him in a climactic struggle. Their physical confrontation ends with Sawyer managing to escape his grasp, yet the haunting fear lingers.
Six months later, Sawyer tries to rebuild her life, even enjoying lunch with Jill, her work friend. But when she overhears a conversation that echoes her past fears involving a man professing love, it becomes evident that the scars left by David are far from healed. In a moment of panic and disbelief, she races out of the restaurant, realizing that despite David's death, his shadow will always loom over her existence.
