In December, after the cancellation of their flight out of Boise Airport to Denver due to stormy weather, neurosurgeon Dr. Ben Bass (Idris Elba) and photojournalist Alex Martin (Kate Winslet) hire Walter (Beau Bridges), a local private pilot, to get them to Salt Lake City for connecting flights to Alex’s wedding in New York (scheduled for the following day) and Ben’s emergency surgery appointment in Baltimore, Maryland. As they board a small plane along with Walter’s dog, Alex asks Walter if he has filed a flight plan, to which he says no, he doesn’t need to, as “… it’s still light out …”, they’re flying VFR, and they’ll beat the approaching storm. He tells them he flew combat missions in Vietnam, so unless somebody is shooting at them, they’ll get there. Minutes after takeoff, Walter points out they’re over the High Uintas Wilderness, ‘a million acres of pure ass nature’. Alex notices the storm is ahead as Walter suffers a stroke, causing the plane to crash on a mountaintop. Ben, Alex, and the dog survive, with various injuries, but Walter is killed.
Ben buries Walter and tends to Alex’s wounds, realizing they must find civilization for any hope of survival. Alex asks Ben to leave her behind as he has a better chance of surviving without her, but Ben refuses. Instead, he climbs a peak to see if there are any structures nearby. On his descent, he nearly falls off a cliff face. Meanwhile, a cougar approaches the plane wreckage and attacks and injures the dog. The cougar then approaches Alex within the plane wreckage but she wounds it with a flare. Ben returns to find a blood trail and rushes to the plane. He treats the dog and later retrieves the cougar’s body and barbecues it for food.
Stranded for days with dwindling sustenance, Ben and Alex find strength in each other. Alex becomes skeptical that help will come, although Ben continues to insist they wait for help. After a heated argument, they both sleep, and Alex resolves that she will leave. At first light, she proceeds to leave the wreckage and starts a descent down the mountain. Ben awakens, gives chase, and eventually catches up with Alex, and they make up over past grievances.
While making shelter in a cave, Alex uses her camera to locate a man-made structure and spots a reflective material flashing at her. They agree to make it down to this structure but on the way, they become separated from the dog. Ben goes after him and finds an abandoned cabin. As he looks through it, Alex feels the ground beneath her cracking. She realizes she is standing over thin ice and she falls through into the ice cold water.
Ben hears the cracking and, seeing Alex fall, runs over, and pulls her out but she is unconscious and almost blue. Ben takes her to the cabin and starts a fire to warm her up. He takes the risk of giving her an injection he resourcefully crafted out of materials in the cabin. A couple of days later, Alex eventually wakes up to the sound Ben makes with the piano in the cabin.
The first time Ben leaves Alex alone in the cabin, she goes through his personal things and again listens to a voicemail recording from his wife. Ben catches Alex in the act. She says she didn’t mean to be nosy, but she just wants to know something about him, especially since he has disclosed so little about himself and he never talks about his wife. He allows Alex to finish listening to the voicemail, which implies that his wife left him. He tells Alex that it’s been years since he last listened to the voicemail.
When Ben tells Alex he is going outside to get more wood for the fire, the culmination of all of their experiences together finally overcomes them. No longer able to resist each other, they have sex. As they lay in bed, Ben finally tells Alex that his wife died two years ago from a brain tumor, and he feels he was unable to save her. As Ben sleeps afterward, Alex takes his picture, having been reluctant to do so before.
Deciding they still won’t make it, Alex again asks Ben to leave her behind to find help and return. Ben initially agrees but has regrets about leaving her behind. He returns once more, and they begin to press forward again. As they make their way down, Alex begins to tire quickly. However, Ben still refuses to leave her behind. They fall asleep in the woods. The dog wakes Ben, as it has found ‘civilization’, a timber yard just down the mountain.
Ben wakes Alex and they make their way towards it, but Ben gets his leg caught in a bear trap. Alex tries to help him, but can’t, so she keeps going, reaches the yard, and collapses in front of an approaching truck. The truck driver comes to her aid.
Ben wakes up in a hospital and goes to Alex’s room, where he finds her with Mark (Dermot Mulroney), her fiancé. Mark praises Ben for saving Alex, to which Ben replies she saved him, and he leaves the room, heartbroken. Some time afterward, Mark tells Alex, “I told myself that if you ever lost a part of you (an arm or leg in her dangerous job as a photojournalist), I would still love you.” But now, it’s obvious to him that he has lost her heart.
They go their separate ways after the hospital, with Ben keeping the dog. Alex tries calling Ben, but he declines her phone calls until he receives pictures of him and Alex on the mountain. This encourages Ben to call Alex.
They meet at a restaurant in Manhattan, where it is revealed that Alex is now a part-time teacher who is uncomfortable flying, and Ben is now working as a consultant at trauma clinics, as his frostbitten hands have not recovered sufficiently for him to operate. Ben admits that he did not call Alex back because he thought she would be married, and Alex responds that she couldn’t go through with it; she fell in love with Ben while they were together on the mountain. After leaving the restaurant, Ben admits to Alex that they survived because they fell in love.
Alex dismisses her feelings and reminds Ben of what he said on the mountain: “the heart is just a muscle.” They hug goodbye, and as they depart in opposite directions, they feel deep regrets and burst into tears and run back to each other’s arms.
