The Housemaid follows protagonist Millie Calloway, a down-on-her-luck woman who has recently been released from prison. After losing her job and apartment, she has been living in her car, and, desperate for work, she secures a position as a live-in housemaid for the wealthy Winchester family. Nina Winchester, the glamorous matriarch, seems warm at first, but Millie quickly notices unsettling dynamics within the household. Her husband, Andrew, appears charming and considerate, but Nina’s behavior is erratic and demanding, and her daughter Cecelia is defiant and spoiled. Millie is forced to stay in a small attic room that locks from the outside and has deep scratch marks on the door, almost as if someone was trying to escape. 

As Millie settles into the household, strange and disturbing things begin to happen. Nina’s behavior becomes increasingly bizarre—she destroys the kitchen, blames Millie for trivial things, and forces her to redo menial tasks. Millie assumes Nina’s volatility is due to mental illness, especially when she learns that Nina had previously been committed to a psychiatric facility for trying to drown her infant daughter. Nina’s cruelty toward Millie seems to escalate intentionally, creating a fraught and hostile environment. Enzo, the Winchesters’ landscaper, tries to warn Millie that she’s in danger. Though Millie is uncomfortable with the strange atmosphere, she feels she has no choice but to stay because of her precarious financial situation.  

Meanwhile, Millie grows closer to Andrew, who confides in her about his marital struggles. Millie eventually begins an affair with Andrew, believing he is trapped in a loveless marriage with a mentally unstable wife. Nina’s behavior devolves until Andrew kicks her out permanently and commits to a relationship with Millie. The next day, Millie wakes up locked in the attic, realizing that Andrew has imprisoned her.  

The narrative switches to Nina’s perspective, and she shares her own story of falling for Andrew’s charm, only to be trapped in a marriage where she was routinely locked in the attic and tortured. When she threatened to leave him, he tried to drown Cecelia and convinced her that she had done it. Nina’s erratic behavior and disheveled appearance, which Millie once judged as signs of privilege and mental instability, are recontextualized as the results of prolonged abuse at Andrew’s hands. Finally, Nina reveals that she hired Millie for a reason. She knew Andrew would try to seduce and then torture Millie, and she hoped Millie would kill him in self-defense, as Mille has a long history of vigilante violence. Nina carefully orchestrated events to push Millie toward an inevitable confrontation with Andrew, even going so far as to have a fertility specialist lie to Andrew about Nina being unable to conceive, knowing that Andrew wanted a younger woman who could give him a child. 

The narrative shifts back to Millie’s perspective. As Andrew’s prisoner, Millie  is forced to perform painful tasks, like balancing books on her stomach for hours. Andrew uses surveillance and manipulative punishments to control her, revealing his sadistic nature. In a climactic confrontation, Millie turns the tables on Andrew. After enduring days of physical and psychological torture, Millie locks Andrew in the attic and forces him to endure the same humiliating punishments he had inflicted on her and Nina. She forces him to pull out his own tooth and balance books on his genitals, taking revenge for the torment she’s suffered. When Nina returns to the house, she finds Andrew’s battered and broken body, with Millie standing over him. Millie breaks down, terrified she’ll go to prison for the rest of her life, but Nina urges her to leave, promising to take the blame for what happened. 

Nina is subsequently questioned by the police but ultimately saved by Detective Connors, whose daughter was previously engaged to, and abused by, Andrew. Connors acknowledges that Andrew’s power over the police kept him immune from suspicion for years. Nina leaves the interrogation free, reflecting on how she’s finally rid of Andrew and the torture she endured. 

The epilogue reveals Millie a year later, living off the money Nina deposited into her account after Andrew’s death. She takes an interview with a wealthy woman, Lisa Killefer, who has a bruise on her arm. Millie quickly realizes that Lisa is trapped in a situation similar to Nina’s and understands why Nina recommended her for the job. Millie is left with a decision: whether to help another woman trapped in an abusive relationship. She accepts the position, officially stepping into the role of protector of battered women.