Karla Dyson, a diner waitress, lives a mostly normal life as a single mother with her 6-year-old son, Frankie, despite her involvement in a custody battle with her estranged husband and her daily issues with the rude customers at her workplace.
One day, Karla takes Frankie to the a park carnival. After temporarily leaving her son to take a phone call, she returns to find that her son is missing, having left his toy voice recorder behind. Frantically searching for her son, Karla spots a woman aggressively dragging Frankie into a green Ford Mustang with no license plate. As the car takes off, Karla desperately clings to the side of the car trying to stop them, losing her phone in the process. Karla then jumps into her minivan and races after the kidnappers.
Karla tries to get help from nearby motorist but her attempts are foiled by Frankie's abductors. Karla is then forced to take an exit ramp after the woman threatens to kill Frankie. Unwilling to abandon her son, Karla resumes her pursuit.
Karla hears a voice from her son's toy voice recorder, revealing the abductor's name to be Margo. Karla then spots a police motorcycle behind her and sways her car from side to side, successfully gaining the police officer's attention. The cop orders Karla to pull over, despite her protests that she is innocent and her son has been taken. The kidnappers ram their car into Karla's, trapping the officer's motorcycle between both vehicles, where he is eventually thrown off onto the highway and killed.
Stopping in a grassy field, Karla confronts the male driver of the green car and demands the release of her son. To her dismay, the man does not respond. Margo then gets out of the car and forces Karla to ride with her, demanding a ransom payment of $10,000 in exchange for her son's release. Margo orders Karla to follow her accomplice's car.
After entering a tunnel, Margo attacks Karla in the driver's seat but she successfully fights back and throws Margo out of the car during the ensuing struggle. Karla puts on Margo's sweater, fooling the second abductor temporarily as she exits the tunnel. After realizing that Margo is gone, the male kidnapper forces Karla to stop tailing him by threatening to drop Frankie onto the highway. After Karla loses them for several minutes, she spots a traffic jam, drives ahead of it, and finds the kidnappers' car abandoned after causing a collision. One of the motorists tells her he saw the man and the boy emerge from the car and Karla drives after them once again.
Karla stops at a police station to report the incident. but, after seeing posters of young children who have been missing for a decade, and fearing that her son will likewise disappear for good, she continues the chase on her own. Karla eventually spots Frankie's abductor - who has now stolen a black Volvo - and chases him until her vehicle runs out of fuel. Karla frantically stops a police vehicle to hitch a ride to follow the kidnapper, but they are blindsided when the Volvo rams their vehicle without warning, killing the driver and knocking her unconscious.
As she comes to, Karla discovers that her son is not in the Volvo. Eventually, the male kidnapper emerges from his car and shoots at Karla with a sawed-off shotgun. As he tries to attack her in the car, she slips her car into reverse, causing it to careen into the woods where the man is fatally struck by a tree. Karla demands to know where her son is, but he dies before she can get the answer. Karla finds his ID card, and learns his name, Terrence Vickey, as well as the address where her son might be.
Karla arrives at the Vickeys' house at nightfall and eventually locates Frankie in the barn with two other kidnapped girls. Before that, she calls 911 on their landline as she hides from Margo, who leaves the house in search of Terrence. Karla successfully rescues Frankie, but when Margo comes back and realizes that Terrence has perished, Karla and her son run out of the barn before she can retrieve the two girls. Karla creates a diversion by sailing a skiff away while hiding underwater.
Margo discovers them hiding, but Karla manages to drag Margo into the water, drowning her and killing the dog when Margo's shotgun is wildly fired. Returning to the barn, she is confronted by a man who claims to be the next-door neighbor and holds her at gunpoint. After hearing children's voices in the barn attic, he then offers to help Karla by getting the two girls down. Karla realizes that he is actually participating in the kidnapping ring, since he knows how many people were hiding without her telling him. She then knocks him out with a shovel just before he is able to draw his gun.
Karla is rescued along with the three children as the police arrive. Media reports praise her for saving the children from the kidnappers, announcing that Karla's actions have led to the dissolution of an international child abduction ring, with arrests being made in other parts of the United States and the world. The media hails Karla as a hero.