The film opens with a striking scene of a car facing a frozen lake, enveloped in smoke. Set in the early 1960s, Eileen (Thomasin McKenzie) finds herself in her vehicle, distracted by a young couple's flirtations in a nearby car. In a moment of impulsive excitement, she shoves snow down her pants before driving back home in her smoky, malfunctioning car. Upon her arrival, she encounters her father, Jim (Shea Whigham), who is slumped in an armchair, surrounded by empty liquor bottles. Eileen dutifully replaces the bottles as a way of maintaining order in their chaotic home. After making her way to her room, she indulges in sweets, only to spit them out before drifting off to sleep.
The next day, Eileen heads to the boy's prison where she works as a secretary. While on duty, she notices Lee Polk (Sam Nivola), a young man isolated from the other inmates. During visiting hours, Eileen performs her responsibilities of patting down the incoming female visitors while engaging in fantasies about a young guard. After work, she witnesses a going-away party for the outgoing prison psychologist, Dr. Frye (Peter Von Berg), who is being replaced the following day.
Eileen’s home life is no sanctuary. She finds Jim drunk in the street, brandishing his gun at passing pedestrians as Buck (Jefferson White), a policeman, attempts to de-escalate the situation. Once inside, Jim criticizes Eileen for failing to live up to the expectations of her late mother and sister. In an effort to keep him restrained, she locks away all of his shoes in her car.
The arrival of Rebecca St. John (Anne Hathaway), the new psychologist, shakes up Eileen's world. Instantly captivated by Rebecca's allure, Eileen discovers they share similar feelings of detachment from societal norms. As Christmas approaches, an annual pageant at the prison becomes chaotic due to unruly prisoners. Afterward, Eileen picks up a pack of cigarettes, reminiscent of those Rebecca had smoked earlier. It's revealed that Lee is incarcerated for killing his father, leaving a trail of silence in the aftermath of the incident.
Eileen's fascination with Lee escalates as she watches him through his cell window and looks through his graphic file depicting the murder. Their paths converge further when she runs into Rebecca, prompting an invitation for drinks at a local bar. Dressed to impress, Eileen heads out and, during the evening, shares intimate moments with Rebecca, culminating in a kiss that leaves her both elated and bewildered.
However, the aftermath of their night out leads Eileen to a grim reality. She awakens locked out of her home, facing the repercussions of her father's erratic behavior. After Buck sees to the complaints regarding Jim's antics, Eileen receives custody of his gun with strict instructions not to use it against him. Struggling with dark thoughts, she finds herself in a downward spiral.
Upon returning to work, Eileen learns that Rebecca will not be back until after the holidays. Distressed, Eileen breaks into Rebecca's office, finding solace in solitude. When she returns home, she finds Jim in dire straits after falling down the stairs, leading to a hospital visit that exposes the severity of his alcohol dependency. Jim’s harsh words about her worthiness trigger an emotional breakdown.
Days later, an invitation to Christmas Eve at Rebecca's home ignites hope within Eileen. The two share revelations over wine, discussing the yearning for 'normalcy,' but the atmosphere shifts dramatically when Rebecca unveils a shocking secret: she has Mrs. Polk, Lee's mother, confined and under duress for her complicity in the abuse Lee endured.
With Eileen manipulated into threatening Mrs. Polk, things spiral out of control. After Mrs. Polk confesses her involvement, a moment of rage compels Eileen to shoot her. The ensuing chaos heightens Rebecca’s panic as they scramble to contain the situation, leading Eileen to propose a cover-up that would suggest Jim was responsible.
As the plot thickens, Eileen's infatuation clouds her judgment. When Rebecca fails to return, Eileen takes matters into her own hands, devising a dark plan that involves putting Mrs. Polk in the running car. The film concludes with Eileen hitchhiking away, a smile of satisfaction indicating a newfound freedom amid her turmoil.
Eileen's transformation from a silent observer to an active participant in violence reflects the complexities of her character, leaving audiences to grapple with the contradictions of love and moral ambiguity.
