Sarah Pierce is a hapless, stay-at-home mother in a small suburb of Boston. She had been working on a doctorate in English, but set aside her work to marry Richard, and raise their 3-year-old daughter, Lucy. Her marriage falls apart when she discovers that Richard is addicted to online pornography. Sarah meets Brad Adamson, a law student who brings his 4-year-old son, Aaron, to the park. Brad is married to Kathy, and although their marriage is loving and amicable, it has been lacking intimacy.
When Brad is supposed to be studying for the bar exam, he instead plays on a local football team or sits and watches teenagers skateboard outside his house, fantasizing about being young and carefree again. Brad and Sarah become friendly and, on a dare, kiss in the park, scandalizing the other park parents. They are instantly attracted to each other, but resolve to keep their relationship platonic. One day, several parents panic when they see sex offender Ronnie J. McGorvey, who was recently released from prison, swimming in the pool with the children. After Ronnie is escorted away by the police, it begins to rain.
Sarah and Brad take Lucy and Aaron back to her house and put the kids to bed. Brad looks at one of Sarah’s books and finds a photo of him in it. While Sarah is drying towels in her basement, Brad kisses her and they have sex. Brad’s friend, Larry Hedges, is a former police officer who was forced to retire when he accidentally shot a teenager at a local mall. Now he is estranged from his wife and spends much of his time harassing Ronnie. Ronnie lives with his mother, May, who believes that meeting a woman his own age would cure him of his pedophilia. Ronnie knows this is futile, but agrees to go on a date May has arranged for him with a woman named Sheila.
During dinner, Ronnie tells Sheila about his criminal record, and Sheila in return tells him that she has had a series of nervous breakdowns. They seem to get on well, but the date ends badly when he has her drive by an elementary school playground so he can masturbate next to her in the car. When Brad skips taking the bar exam again, Kathy grows suspicious and tells Brad to invite Sarah, Richard, and Lucy over for dinner. 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. When Brad’s football team plays its final game, Sarah attends and cheers as Brad scores the winning touchdown.
Afterwards, while Larry waits for Brad at a nearby bar to celebrate their victory, Brad and Sarah make out on the field. He admits that this is the happiest moment of his life, and convinces Sarah to run away with him. Larry goes to Ronnie’s house and uses a bullhorn to taunt him. May comes out to confront him and has a heart attack. Larry is arrested and May is taken to the hospital. While Ronnie sleeps in the waiting room, May dies. When Ronnie goes home, he finds an envelope containing a letter written by his mother saying: “Please be a good boy.” Distraught, Ronnie destroys much of his mother’s collection of Hummel figurines, then takes a butcher knife from the kitchen.
That same night, Sarah and Brad agree to meet in the park 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. Before he can get to the park, he is distracted by skateboarding teenagers, who convince him to try a jump himself. Brad does so, but falls and knocks himself out. When he regains consciousness, he asks the paramedics to call his wife and 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.
When Sarah takes Lucy to the park, she sees Ronnie stagger by, and briefly feels afraid. When she sees him crying about his mother’s death, however, she feels sorry for him. When Lucy disappears, Sarah panics and rushes to find her, forgetting about Brad. After Sarah finds Lucy and puts her in the car, Sarah starts crying, realizing her getaway with Brad is just a fantasy. Larry is upset about having indirectly caused May’s death. He genuinely wants to apologize to Ronnie, but when he finds Ronnie in the park, he discovers that he has castrated himself and is bleeding to death. Larry races him to the hospital, and they arrive just as Kathy meets Brad at the emergency room doors.