Sarah is a middle-aged woman who lives in rural Massachusetts, wife to Jim and mother of a young baby. One day, she goes out to do some gardening, when she encounters a large dog, terrified and sprinting. The dog is being shot at, by a teenager in the passenger seat of a blue Porsche. Enraged at witnessing animal cruelty, Sarah picks up a rock and throws it, shattering the Porsche's rear window. The Porsche drives away but then circles back, and Sarah knows they want payback.

First, she runs into the woods and hides, but returns when she realizes they aren't behind her. She sees the Porsche parked in her driveway and her front door ajar. She doesn't have her cellphone on her and is considering running to get help when she hears gunshots and Jim crying. She realizes she has to protect her family.

Next ensues a high-paced action scene. Sarah slinks around to the shed, where she picks up a bottle of wasp spray, a fishing knife, and an old nail gun. She leaves the shed and runs into one of the boys, whom she sprays with wasp spray and shoots with the nail gun. One nail goes into his forehead and he dies immediately. Sarah feels no remorse, but instead is consumed with the need to protect her family. The other boy is horrified, and holding a gun to her head, forces her to come inside. There, he goes from room to room, shooting anything he deems valuable to her including a piano, antiques, and computers.

While he reloads his gun, Sarah hits him with a lamp and takes the gun, though it is still unloaded. They run outside, and she threatens to smash his Porsche more. She runs away, but he catches her. Suddenly, she sinks her fishing knife into his back and he dies. Sarah realizes she has killed two boys, all just from one rock thrown at a car, but doesn't feel remorse. She says the boys got what they wanted: her image of herself as decent and moral has been shattered. But protecting her family successfully — that's her payback.