Plot

Edmond Murray searches for answers deep in the Highlands after receiving a frantic call from his ex-wife that their seven-year-old son went missing from a campsite.

Edmond Murray (James McAvoy) is an absent father who is called to the site of his son Ethan's disappearance. He meets and briefly consoles his ex-wife Joan (Claire Foy) there, and promptly joins a search party before being interviewed by the police.

After being interviewed by the police, Edmond visits his ex-wife's new partner, Frank, and the two begin talking. After seeing that Frank has made plans for a new home for himself and Joan (without bedroom accommodation for Ethan), and upon hearing that Frank has given Joan a Valium tablet to help her sleep, Edmond becomes irate and confrontational, accusing Frank of being responsible for his son's disappearance. He assaults him in the kitchen, knocking him unconscious and calls Inspector Roy to have him look into Frank.

Edmond is arrested and no charges are filed, as Frank is apparently forgiving of Edmond due to his emotional stress at the moment. Edmond is later met by Inspector Roy, who hides Edmond's phone in a microwave oven (so as not to be heard), and informs him that he has been taken off the case without any explanation.

Edmond looks through Frank's phone, which he took after knocking him out, and finds videos of Ethan leading up to the disappearance. During this, he realizes there is a car present in both videos, two weeks apart, hidden barely out of sight. Relying on Joan's brother who works in the insurance industry, Edmond obtains the vehicle owner's address.

After arriving at the location, he hears a vehicle approach. Hiding, Edmond grabs a crowbar and eventually knocks out the stranger, and ties him up to a post in the shed. After an aggressive interrogation involving a blowtorch, the stranger reveals that he is responsible for sending pictures of children outside of schools to interested kidnappers, who then "choose" who will be kidnapped. After being told of his son's location, Edmond rushes uphill to a lodge, where three captors are inside.

Edmond incapacitates their vehicle, knocks out one of their cronies, finds his son and relays his location to Joan and Inspector Roy. After a narrow escape, Edmond is shot and veers off the road. Joan and Ethan exit, hiding behind a tree before being found by Roy, who takes them to safety.

Outdoors and surrounded by water and mountains, Joan and Edmond stand with Ethan as he flies an RC quad copter. Roy tells Edmond that it is time for them to leave, and he bids his son farewell. As the film ends, Roy mentions that the judge will likely take into account the benefit of Edmond's actions in court cases for criminal charges still pending, as there is an even greater network of kidnappers soon to be arrested.