In Seattle, Washington, a pregnant housewife named Claire Bartel attends a routine appointment with her new obstetrician, Dr. Mott, and he sexually molests her. Reeling from the assault, Claire has an asthma attack, but she revives with an inhaler. With the encouragement of her husband, Michael, Claire files a legal complaint against the physician, and several other women come forward. However, Dr. Mott commits suicide before going to trial, leaving his wife pregnant and penniless. The stress causes Mrs. Mott to miscarry, and she vows to exact revenge on Claire. Six months later, Claire is busy caring for her newborn son, Joe, and her young daughter, Emma. When the girl asks her parents to hire their handyman, Solomon, as a nanny, Claire explains that the man has a mental disability and cannot be trusted, despite his dependable good nature. That afternoon, Dr. Mott’s wife arrives at the Bartel home, looking for nanny work, and introduces herself as “Peyton Flanders.” She ingratiates herself and elicits Claire’s sympathy by confessing that she is a widow, who suffered a miscarriage after her husband’s death. Oblivious to Peyton Flanders’s true identity, Claire hires the woman as her live-in governess. As Peyton moves into the Bartel home, she gives the family a wind chime and presents herself as a humble people-pleaser. Her quiet demeanor lulls them into a false sense of security while she chips away at the family unit. Peyton secretly breastfeeds baby Joe, making him disinterested in his mother’s milk, and creates an exclusive bond with Emma. Feeling idle and insignificant, Claire helps Michael complete an important grant proposal for his scientific research and offers to deliver it to the air shipping company, but Peyton secretly destroys the document and Claire blames herself for misplacing it. The incident prompts an attack of Claire’s chronic asthma, and creates marital tension with Michael. One day, the simpleton handyman, Solomon, sees Peyton breastfeeding baby Joe and the young woman conspires against him, planting a pair of young Emma’s panties in his toolbox. Believing Solomon is a pedophile, Claire suffers another asthma attack and fires the handyman. However, Emma protests the dismissal of her friend and seeks solace from Peyton, who learns from the girl that Michael had a childhood romance with the couple’s best friend, Marlene. Sensing another opportunity to destroy the family, Peyton encourages Michael to plan a surprise birthday party for Claire, and to plan the event with Marlene. After the friends meet, Peyton slips Marlene’s cigarette lighter in Michael’s coat pocket, and Claire believes the misplaced personal belonging is evidence of an affair. Claire accuses Michael of infidelity, unaware that her shouting is audible to a crowd of partygoers hiding in the living room. Humiliated and bewildered, Claire begins to doubt her sanity and asks Michael for a family holiday. However, Peyton overhears the plan and becomes more aggressive. Plotting murder, Peyton rigs Claire’s greenhouse so the roof will collapse, but the plan is waylaid when Claire makes an unexpected trip to her volunteer job at the botanical gardens. Meanwhile, Marlene returns to her real estate office and scans old listings. Looking at a photograph Dr. Mott’s former home, Marlene notices the same wind chime Peyton gave the Bartels and realizes Claire is in danger. At the Bartel home, Marlene demands to see Claire, and Peyton guides her to the greenhouse, knowing the woman will be killed in her trap. Predicting that Claire will have an asthma attack upon finding her dead friend, Peyton empties her boss’s inhalers and Claire nearly chokes to death. As Claire convalesces in a hospital, Peyton attempts to seduce Michael at home, but he turns her away. Shortly thereafter, Claire leaves the hospital and goes to Marlene’s office, where she discovers the Mott home listing. Claire tours the estate as a potential buyer and notices that the nursery is decorated with the same trim Peyton used in baby Joe’s room. Realizing Peyton is really Mrs. Mott, Claire speeds home, punches the nanny in the face, and orders her to leave, but Claire fears Peyton will return. Although Michael telephones the police, the authorities do not believe the situation is an emergency, and the family decides to go away for the weekend. Before they leave, however, Peyton comes back and knocks Michael down the stairs with a shovel, rendering him immobile. As Claire calls police, Peyton beats her unconscious and goes after Emma, referring to herself as “Mommy.” The girl is no longer deceived by her nanny’s false charm and locks Peyton in the nursery, but the frenzied woman breaks through the door and follows baby Joe’s cries to the attic. There, Solomon reappears after secretly patrolling the family from afar and attempts to rescue the children. As Peyton threatens Solomon, Claire awakens and fights the nanny, then feigns an asthma attack so Peyton will believe she has an advantage. While Claire gasps for air, the nanny charges toward baby Joe, but Solomon shields the child and Claire springs into action, pushing Peyton out the window. With Peyton dead and ambulances on the way, Claire embraces her children and praises Solomon, who is welcomed back into the family fold. Claire asks Solomon to look after Joe while she tends to Michael, and the handyman is delighted by his new job as the baby’s caregiver.