Pregnant attorney Caitlyn Morales [Mary Elizabeth Winstead] is assigned to represent Polly Murphy [Maika Monroe], a young woman facing eviction and homelessness. Shortly after their first meeting, Caitlyn gives birth to her second daughter, Josie [Lola Contreras], while juggling the demands of motherhood, a demanding career, and her volatile older child, Emma [Mileiah Vega]. When the women meet again, Polly asks for employment as the family's nanny, and she quickly becomes indispensable, charming both Emma and Caitlyn's husband, Miguel Morales [Raúl Castillo], with warmth and attentiveness. Yet beneath Polly’s poised exterior lurks something more sinister.
She secretly poisons Caitlyn and the children by spiking a seafood stew, making their illness seem accidental. Grateful for Polly’s continued help, Caitlyn and Miguel invite Polly to live in their guest house, allowing her to discreetly overstep boundaries. Polly discovers that Caitlyn takes medication for an unspecified mental condition and, cooking up a more terrible scheme, replaces the pills with carved methamphetamine tablets, gradually destabilizing Caitlyn’s grip on reality. Paranoia, irritability, and jealousy about Polly’s growing influence over Emma begin to erode Caitlyn from within.
When Emma accidentally ignites fireworks that Polly had supplied, Caitlyn lashes out and fires Polly, despite Miguel’s pleas for mercy. Wracked with guilt and desperate to keep Emma from further zealotry toward Polly, Caitlyn tracks Polly to a dilapidated motel and rehires her. She also enlists her neighbor, Stewart [Martin Starr], to investigate Polly’s past, and contacts a supposed former employer listed as a reference, learning that Polly had met Polly’s own fabricated story of abuse at an Alcoholics Anonymous meeting.
Stewart confronts Polly with the results of his background check, revealing that Polly’s true name is Rebecca, and that both she and Caitlyn—whose real name is Jennifer—share a dark, intertwined history. Years earlier in San Bernardino, a fire set by young Jennifer had destroyed Rebecca’s home, killing her parents and infant sister. Jennifer was given a new identity, while Rebecca endured years of trauma in abusive foster homes. Although Stewart urges forgiveness, Polly kills him with a baseball bat, stealing his research before fleeing.
Caitlyn learns of Stewart’s assault and rushes home, only to find Polly calmly dining with Miguel and the children while dressed in Caitlyn’s clothes. Enraged, Caitlyn destroys Polly’s belongings and lunges at her, accidentally striking Emma during the confrontation. Miguel, horrified, expels Caitlyn from the house and files domestic-violence charges, barring her from seeing her daughters unsupervised. Desperate, Caitlyn later appears at Emma’s basketball game and pleads for reconciliation. Emma forgives her and secretly returns Stewart’s documents which she had stolen from Polly. A childhood photograph of Polly triggers Caitlyn’s memory, and she recognizes Polly as the surviving child from the fire.
Racing home, Caitlyn finds Polly fully transformed into her double, even mimicking her hairstyle and mannerisms. The two share a tense kitchen showdown. Caitlyn confesses that she set the fire not out of malice but to escape Polly’s father, who had sexually abused both girls. Polly doubts her sincerity but acknowledges that, true or not, Caitlyn robbed her of a family and any chance at happiness. Caitlyn attempts to embrace Polly, but Polly shatters a glass and stabs her, declaring her intent to assume Caitlyn’s life and raise her children as her own. A violent struggle follows, culminating with Caitlyn pushing Polly through a plate-glass window. As Caitlyn flees with Josie, Polly attacks their car, sending Caitlyn into oncoming traffic. The crash leaves Caitlyn and Josie dazed, while Polly sustains fatal injuries. Witnessing the commotion, Miguel arrives with Emma and apologizes for doubting Caitlyn. Caitlyn holds Polly in her final moments.
Later, Caitlyn stares into a lit fire, haunted but alive. Nearby, Emma imitates Polly’s gestures and repeats one of her stories about life in foster care, soothing her baby sister.
