Margaret is a composed, accomplished businesswoman residing in Albany, New York, juggling her professional life with her role as a single mother to Abbie, who is preparing to leave for college. In the midst of her busy life, she engages in an affair with her married colleague, Peter. However, the stability of their lives begins to unravel when Abbie discovers a tooth in her wallet and later suffers a biking accident while intoxicated.
During a work conference, Margaret runs into David, a figure from her past, which triggers a panic attack that leads her to return home and limit Abbie's social interactions. Conflicted by her memories, she experiences a nightmare about an unusual baby in the oven, leaving her feeling increasingly irritable. Desiring intimacy, she attempts to seduce Peter at work but soon encounters David again during a shopping trip. Overwhelmed, she dashes away with Abbie, who is growing more suspicious of her erratic behavior. When confronted, David ominously suggests that "Ben is with me," flashing a smile that reveals his missing tooth.
Desperate for help, Margaret approaches the police but is met with indifference. Feeling vulnerable, she secures her home with new locks and even purchases a gun. Flashbacks reveal her troubled history with David, who she met at just 18. Initially charming, David's true nature emerged as he became abusive and controlling, demanding "kindnesses" in the form of self-harm as proof of her love. After the birth of her son, Benjamin, jealousy spurred David to commit unspeakable acts, leaving Margaret emotionally shattered. After finding only two fingers upon returning home one day, she spiraled into catatonia as David further manipulated her beliefs about their son.
As the story unfolds, Margaret finds herself in a web of psychological torment, with David insinuating his involvement in Abbie's accident. He tempts her with a key to his hotel room, where she discovers a haunting relic — Ben's baby blanket — and wakes up in distress, strangely lactating. Driven to desperation, she devises a plan to confront David near a river, evoking concern from both Abbie and Peter, who urge her to seek help. When her attempt to kill David fails, she is overpowered. In her exhaustion and delusion, she imagines hearing Ben's cries and succumbs to David's demands for another "kindness": a bizarre public yoga position at night.
The next morning, a frightened Abbie flees from Margaret, prompting Peter to confess his feelings for her. Instead of reciprocation, she responds with hostility, physically confronting him. David tracks Margaret down at work, suggesting another rendezvous at a hotel room. In a moment of despair, Margaret writes a goodbye letter to Abbie and films a farewell video.
In a chilling confrontation in another hotel room, David forces Margaret to feel his belly, insisting that Ben is inside him. Overcome by rage and despair, she retaliates, seizing knives to confront him. In a brutal climax, she ties him to the bed and slices open his abdomen, revealing what she believes to be a breathing fetus.
The conclusion sees Abbie embarking on her college journey, filled with affection for her mother. As Abbie thanks Margaret for her protection, a tender moment unfolds with Margaret holding the baby. Yet, her expression shifts from joy to horror as she gasps in terror, leaving viewers in shock at the haunting outcome of her tumultuous story.
