In
Los Angeles, pregnant attorney Caitlyn Morales is assigned to represent Polly Murphy, a young woman facing eviction and homelessness. Shortly after their first meeting, Caitlyn gives birth to her second daughter, Josie, while struggling to balance motherhood, career, and her volatile older child, Emma. When the women meet again, Polly asks for employment as the family's nanny. Polly quickly becomes indispensable, charming both Emma and Caitlyn's husband, Miguel, with her warmth and attentiveness. However, beneath her composure lies malice. She secretly poisons Caitlyn and the children by spiking a seafood stew, ensuring their illness appears coincidental. Grateful for her continued help, Caitlyn and Miguel invite Polly to live in their guest house, allowing her to subtly overstep household boundaries. Polly discovers that Caitlyn takes medication for an unspecified mental condition. Replacing the pills with carved
methamphetamine tablets, she gradually destabilizes Caitlyn's mental state. Paranoia, irritability, and jealousy over Polly's growing influence on Emma begin to consume her. When Emma accidentally ignites fireworks that Polly had provided, Caitlyn lashes out and fires her, ignoring Miguel's pleas for leniency. Wracked with guilt and fearful of alienating Emma, Caitlyn tracks Polly to a dilapidated motel and rehires her. Meanwhile, she enlists her neighbor, Stewart, to investigate Polly's past. Caitlyn also contacts a supposed former employer listed as a reference, learning the woman had met Polly at an
Alcoholics Anonymous meeting and had been deceived by her fabricated story of abuse. Stewart confronts Polly with the results of his background check, revealing that her true name is Rebecca, and that both she and Caitlyn—whose real name is Jennifer—share a dark 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 and a second chance, 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 struggle. 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 stole 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 replicating her
hairstyle and mannerisms. The two share a tense confrontation in the kitchen. 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 her, 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, causing Caitlyn to swerve into oncoming traffic. The crash leaves Caitlyn and Josie dazed, while Polly suffers fatal injuries. Witnessing the event, Miguel arrives with Emma and apologizes for doubting Caitlyn. She 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. ==Cast==