Sarah Pierce (Kate Winslet) is a 30-year-old, stay-at-home mother in a small Massachusetts suburban community. She had been working on a doctorate in English, but once she married Richard (Gregg Edelman) and had their daughter, the presently five-year-old Lucy (Sadie Goldstein), she set aside her research. Now she spends her days taking Lucy to a local park along with three other stay at-home mothers: the severe and judgmental Mary Ann (Mary B. McCann) and more timid Theresa (Trini Alvorado) and Cheryl (Marsha Dietlein). They enjoy ogling from a distance Brad Adamson (Patrick Wilson), the handsome and well built father who brings his son, Aaron (Ty Simpkins), to the park. When their children use the same swing set, Sarah and Brad have an opportunity to talk to one another, titillating the other women. But when they, on a lark, hug and kiss, the women immediately take their children out of the park and refuse to associate with Sarah anymore.
Both Brad and Sarah have unhappy home lives. Brad has yet to pass the Massachusetts state bar exam and doesn't even want to be a lawyer. However, his bossy wife, Kathy (Jennifer Connelly), pushes him to succeed. She makes documentaries for public television for a living. They live a bit beyond their means, forcing them to accept financial support from Kathy's mother. Their sex life is non-existent because Kathy is too self-involved in her career and being a mother. When he is supposed to be studying for the bar exam, Brad instead sits and watches teenagers skateboard outside his house, fantasizing about being young and carefree again. One night, an acquaintance, Larry (Noah Emmerich), persuades Brad, who played quarterback on his high school football team, to join his amateur football team, the Guardians.
Larry is a former police officer forced to retire a few years earlier when he accidentally shot a teenager who robbed a store. Now he is estranged from his wife and spends much of his time harassing Ronnie McGorvey (Jackie Earle Haley), a freaky neighbor recently released from prison after serving several years. Ronnie had been incarcerated for exposing himself to a minor and has a long criminal record for sexually molesting young girls. Brad does not feel comfortable with Larry's campaign against Ronnie to force him to move from the neighborhood, but rarely tries to prevent him from carrying it out.
Sarah's marriage to Richard is as sexless as Brad's is to Kathy. In this case, however, it is because he is addicted to Internet pornography. One day she catches him masturbating in his office and they begin to sleep separately. She buys a flattering swimsuit and begins to attend the public pool because she knows she will see Brad there. They begin a deep if platonic relationship and their children become friends as well. Brad is drawn to Sarah's interest in him, even though he does not find her particularly physically attractive. Sarah craves being sexually desired by someone as conventionally handsome and masculine as Brad. One day, when they have to leave the pool due to a sudden rainstorm, they return to Sarah's house. Brad discovers a photo of himself tucked away in a collection of Shakespearean sonnets. Unable to contain his desire for each other, they have rough sex in the basement while their children sleep upstairs. Thus their emotional infidelity becomes physical as well.
Meanwhile, Ronnie lives with his mother, May (Phyllis Somerville), who has taken him in after his release from prison. She believes that if he were to find the right woman, his sexual desire for children would disappear. Aware of his mental problems with the fascination for young girls, Ronnie knows this is futile but agrees to go out on a date May has arranged for him with a woman in another town, Sheila (Jane Adams). During dinner at a local restaurant Sheila and Ronnie seem to make a connection and she reveals her own emotional demons. However, the date ends badly when he asks her to park alongside the road, then begins to masturbate while scolding her not to tell on him.
Elsewhere, Sarah and Brad grow closer. He skips taking the bar exam so they can have a romantic getaway together. Kathy grows suspicious and tells Brad to invite Sarah, Richard and Lucy over for dinner so she can meet them. The intimacy evident between Brad and Sarah confirms her suspicions, and Kathy arranges for her mother to come for an extended visit so Brad and Sarah can't see one another anymore. However, when Brad's football team plays its final game, Kathy and her mother stay home feeling that their suspicions about Brad are nothing. Sarah attends and cheers as Brad scores the winning touchdown in the team's only victory of the season. Afterwards, while Larry waits for Brad at a nearby bar to celebrate their victory, Brad and Sarah make out on the field. He admits the moment is his happiest ever and asks Sarah to run away with him. She agrees.
Hurt that Brad has stood him up, Larry goes to Ronnie's house and wakes the neighborhood up by using a bullhorn to taunt Ronnie. The neighbors come out to ask him to stop but May confronts him angrily. In the process, she suffers a severe heart attack. Larry is arrested for assault and battery, and May taken to the hospital. While Ronnie sleeps in the waiting room, May dies. Her last act is to write a note to Ronnie: "Please be a good boy." Ronnie is distraught and despondent. He goes home and destroys much of his mother's collection of Hummel figurines, then takes a butcher knife from the kitchen.
That very same night, Sarah and Brad agree to rendezvous in the park where they first met to run away together. Brad tells Aaron he loves him before putting him to bed, writes Kathy a note explaining why he is leaving her, then sneaks out while she and her mother finish the dishes. But before he can get to the park, he is distracted by the skateboarding teenagers again. They convince him to try a jump himself. Unable to resist the rush of wanting to re-live his youth, Brad does so but crashes and knocks himself out. When he comes to, he asks the ambulance driver to call his wife -- Kathy -- to meet him at the hospital. It turns out that he never left the note for her and tells one of the skateboarders to dispose of it for him.
Meanwhile, Sarah takes Lucy to the park. She is shocked to see Ronnie stagger in. When Lucy briefly disappears, Sarah is frightened into realizing that leaving Richard for Brad would be a terrible mistake. She takes Lucy home and puts her to bed and decides to stay.
Larry is upset about causing May's death. He genuinely wants to apologize to Ronnie and finds him in the park where Sarah left him. He notices blood on the ground. Ronnie has castrated himself and is bleeding to death. Larry picks him up and races him to the hospital. He knows Ronnie has done bad things in the past but also recognizes that doesn't mean he has to do bad things in the future. They arrive at the hospital at just as Kathy meets Brad's ambulance at the emergency room doors.