.Hannah Faber, a smokejumper, is struggling after failing to prevent the deaths of three young campers and a fellow smokejumper in a forest fire. She is now posted in a fire lookout tower in Park County, Montana.

Owen Casserly, a forensic accountant in Florida, learns about the death of his district attorney boss and his family in an apparent gas explosion; believing that their deaths were actually a contracted killing and that he is the next target, Owen goes on the run with his son, Connor. He intends to seek refuge with his brother-in-law, Ethan Sawyer, a Deputy Sheriff in Park County. Reaching Montana, they are ambushed by the assassins "Jack" and "Patrick", who force them off the road and down a cliff. Trapped in the car, Owen gives Connor the evidence against the assassins' employer. Connor flees before the assassins kill Owen.

As Ethan discovers Owen's car wreck, Hannah stumbles upon Connor while out on patrol. She takes him back to the tower to contact help. The assassins meet their boss, who instructs them to hunt down and kill Connor. As a diversion to preoccupy the police, they use flares to start a forest fire. They go to Ethan's isolated house searching for Connor and find and interrogate Ethan's pregnant wife, Allison. When they force her to call Ethan, she gives him a duress code; she then fights off the assassins and escapes.

Hannah discovers that a lightning strike disabled the radio at the lookout tower. She attempts to take Connor into town on foot, but they are forced to turn back when the fire blocks their path. Following the radio call, Ethan returns home with the Sheriff, aiming to protect Allison. However, the assassins ambush them; they kill the Sheriff and force Ethan to guide them through the woods.

The assassins arrive at Hannah's fire tower and force Ethan to search it as they observe from a tree. Hannah and Connor hide while Ethan attempts to make it appear as if the tower is empty. After Patrick sees that Ethan seems to be talking to someone, the assassins shoot up the tower. Ethan is injured, but Hannah and Connor manage to escape. The assassins try to follow them but are stopped when Allison, having tracked them down, starts shooting at them. Patrick and Jack split up, with Patrick pursuing Hannah and Connor while Jack stays behind to fight Allison; Allison gains the upper hand and kills Jack.

Connor runs ahead while Hannah fights Patrick but returns after Patrick threatens to beat Hannah to death. As Patrick is about to kill Connor, Hannah severely injures Patrick with a climbing axe and leaves him to burn to death in the approaching fire. Allison reunites with Ethan in the tower, but the fire traps them as they put on oxygen masks and hold each other. Hannah and Connor jump into a stream and watch from under the water as the fire engulfs the forest.

In the morning after the fire has burned out, Hannah's old smoke-jumping team arrives and rescues her, Connor, and Allison. Ethan has died from his gunshot wounds. Connor later prepares to give his father's evidence to the media. Hannah promises to help him through his uncertain future.