The movie opens with a house burning and a little girl watching in terror. We then meet Caitlin, a real estate lawyer, who is heavily pregnant and stressed. She meets Polly, a woman struggling financially, and Caitlin promises to help with her landlord. After Caitlin gives birth to her second child, Josie, she struggles to balance work and family life. She reconnects with Polly, who offers to babysit, and Caitlin, after discussing with her husband Miguel, decides to hire her. Polly comes recommended by 'Rosanna', and quickly starts bonding with Josie and Emma.

Polly subtly manipulates situations, like suggesting Caitlin ask her to take Emma to a basketball game. She bonds with Emma, sharing stories and feeding her a cupcake against Caitlin's wishes. Polly secretly spikes the family's food, causing them food poisoning and landing Josie in the ER. A comforted Caitlin, feeling guilty, asks Polly to move in with them, despite Miguel's concerns about Caitlin's past mental health issues.

Polly moves in, and her behavior becomes increasingly unsettling. She ignores a crying Josie and feeds her formula despite Caitlin's instructions. Caitlin discovers Polly with a friend, Amelia, and later sees them intimately. Caitlin's anxiety worsens, and she realizes her antidepressant pills are causing strange side effects. Polly influences Emma, leading to an argument between Emma and Caitlin about Emma's sexuality. Miguel suggests Caitlin is jealous of Emma's bond with Polly. Unbeknownst to Caitlin, Polly has been replacing her medication.

Polly gives Emma fireworks, leading to an incident that causes Caitlin to kick Polly out. Miguel defends Polly, but Caitlin believes she was deliberate. Caitlin later apologizes to Polly, reinstating her. Caitlin's friend, Stewart, becomes suspicious of Polly and investigates her background. He discovers Polly's real name is Rebecca and that Caitlin's real name is Jennifer. Stewart also finds a newspaper article about a fire that killed a family, implying a connection to Polly and Caitlin's past in San Bernardino. It's revealed that Caitlin (Jennifer) started the fire that killed Polly's (Rebecca's) family years ago due to her father's abuse.

Stewart tries to convince Polly to forgive Caitlin but is then brutally attacked by Polly, leaving him severely injured. Caitlin rushes to the hospital and suspects Polly. Upon returning home, she confronts Polly, but Miguel dismisses her concerns, leading to Caitlin accidentally hitting Emma during a scuffle with Polly. Caitlin, guilt-ridden, leaves and discovers her medication was replaced with methamphetamine. Emma reveals she knows Polly is an orphan and believes Polly wants to be part of their family. Caitlin confronts Polly, calling her Rebecca, and reveals that Polly's father also abused her. Polly, enraged, stabs Caitlin with broken glass, expressing her desire to take over Caitlin's life.

A fierce fight ensues between Caitlin and Polly. Caitlin manages to incapacitate Polly and escapes with Josie. Polly pursues them, smashing the car windshield with a hammer just as Miguel and Emma arrive. A car crash occurs, and Polly dies from her injuries. Caitlin is overcome with guilt. Later, Caitlin is still haunted by the past. Emma tells Josie the story of 'Rebecca,' highlighting that even though Polly is gone, her influence, and the trauma she brought with her, still linger.