Chloe (Amanda Seyfried) is a young woman whom is discussing her business as a call girl to the viewers. She explains that she must be capable of pleasing her clients with her words, as well as with her actions.

Catherine (Julianne Moore), a gynecologist living in Toronto, is working in her office. She glances out of her window at different times to see Chloe entering the hotel down the street from her office with her clients. Catherine's husband David (Liam Neeson), a college professor, lectures to a class.

Catherine hosts a surprise birthday party for David, who has not yet arrived. Assuring the guests that David is just minutes away, she gets a call from him saying that he missed his flight and will not make it home in time for his birthday. While David is on the phone, one of his female students is shown to be at his side.

The next morning, Catherine wakes up to find David at home. He apologizes to her, saying that he didn't know there was a party for him. Though it is an hour before David normally leaves for work, he tells Catherine he that must leave early so he can get coffee. Catherine finds her 17-year-old son Michael (Max Thieriot) with his girlfriend, who is walking around the house in her underwear. Disturbed, Catherine asks if they're using protection. Michael says that David knows about his girlfriend spending the night. The relationship between Catherine and Michael seems strained. Catherine suspects David of having an affair after she sees a picture of him and the female student hugging as part of a text message saying, "Thanks for last night" (the night of David's birthday when he said he missed his flight).

That evening, David and Catherine go out to dinner with two friends. David flirts with the waitress, clearly upsetting Catherine. Catherine goes to the restroom and enters a stall. Overhearing a woman crying in the next stall, she asks her if everything is okay. The woman replies evasively, saying that there is no toilet paper. Catherine hands her some under the partition. The woman states that "men are such assholes." Catherine agrees, then exits her stall. The woman (Chloe) follows her.

As they are washing their hands, Chloe offers Catherine her hairpin. Catherine refuses, kindly, then says she must return to her husband. During dinner, Catherine notices Chloe, still upset, with another client at a table a short distance away.

The next day after work, Catherine stops by the hotel bar where Chloe spends the majority of her time waiting for potential clients. Chloe, looking at Catherine passionately, tells her that she "doesn't usually meet with women." Catherine reassures Chloe, explaining that she suspects her husband of infidelity. She wants Chloe to go to David's coffee shop in the morning and meet with him in order to test his loyalty.

The next evening, Catherine and Chloe meet at a bar. Chloe tells Catherine that she asked David to kiss her, and he did. Angered, Catherine tells Chloe that that wasn't what she wanted her to do. However, she insists that Chloe meet with David again.

The next evening, Chloe tells Catherine that she went with David to a garden, where she seduced him. Catherine is heartbroken. Once more, she insists that Chloe meet with David again.

The next day, Catherine receives a text message from Chloe at work, asking her to meet her at a hotel. When Catherine arrives at the hotel room, the bathrobe-clad Chloe tells her that she had sex with David. She describes the encounter in explicit detail, but rather than causing Catherine more heartbreak, it arouses her. Chloe, feeling remorse, kisses Catherine, who abruptly leaves shocked by this sudden display of affection.

At home, David text messages someone and pays little attention to Catherine. That evening, Catherine goes to meet with David at a music recital where Michael is playing the piano on stage to an audience. Driven somewhat crazy by the haunting music and by both David and Michael's responses at ignoring her, Catherine leaves before the recitial is over.

The upset Catherine meets Chloe at the hotel and has sex with her. Afterwards, Catherine and Chloe share a taxi, with Catherine paying for Chloe's trip home. As Catherine gets out of the taxi, Chloe offers her her hairpin again. She says it was her mother's, and she wants Catherine to have it as a token of her love for her. Catherine doesn't reply to Chloe or take the hairpin.

Catherine enters her home to find David waiting for her. He asks her if she's been unfaithful. Catherine becomes infuriated and tells him she's been meaning to ask him the same question. David declines to respond.

The next afternoon, Catherine arrives at her office after lunch to find several missed calls from Chloe and an e-mail with a picture of them in bed together from that night. Alarmed and embarrassed, Catherine calls her. Chloe tells Catherine that she is waiting in her waiting room. Chloe suddently enters Catherine's office with flowers, and asks her if she enjoyed the sex the night before. Embarrassed, Catherine admits that she did, but explains that their business is finished. Heartbroken, Chloe tells Catherine that she loves her. Catherine, disturbed, apologizes to Chloe for miscommunicating. She thanks Chloe for helping her, and pays her for the work she has done. Chloe leaves Catherine's office crying.

Seeking revenge, Chloe goes to Michael's hockey practice. They flirt, and Chloe gives him a present of a CD.

That night, Catherine and David go to a coffee house, where Catherine demands that David admit that he is having an affair. David admits that he lied to her that night about missing his flight home because he hated to go to the birthday party, but he denies that he is having an affair. Chloe suddenly walks in, shocking Catherine. David looks around to see at whom Catherine is staring. He apparently does not know Chloe when, in a genuine baffled tone, he asks who the girl is, and why Catherine is so upset. At this point, Catherine realizes that Chloe made up her encounters with David to come between Catherine and David. Catherine also realizes that Chloe has apparently fallen in love with her because Catherine gave her attention.

Catherine apologizes to David for not having trusted him, and admits that she worried about him because she was getting older and more insecure about her appearance. Then she confesses her sexual encounters with Chloe. David, shocked but understanding, embraces and kisses her.

Jealous, Chloe goes to Catherine's home. Catherine arrives to find Chloe and Michael in Catherine's bed haver having sex. Angry, Chloe accuses Catherine of thinking that she could invite someone into her life, use them, then order them away when she's finished... just like all of her other male clients. Chloe tells Catherine that "money isn't enough to make her go away". Catherine apologizes and asks Chloe what she wants. Chloe asks for one last kiss as well as to take her hairpin yet again. Catherine kisses Chloe just when Michael walks back into the room. In a fit of anger, Catherine pushes Chloe against a large window which breaks. Catherine then is forced to watch, helpless, as Chloe falls through the bedroom window to her death on the patio below.

Several months later, at Michael's high school graduation party, Catherine obviously is still upset by Chloe's death. One of her guests, sympathetic, gives Catherine a hug. The camera pans around to reveal Chloe's hairpin in Catherine's hair.