Ralston continues into Blue John Canyon, through a narrow passage where boulders are suspended, wedged between the walls of rock. As he descends, one boulder is jarred loose, falling after Ralston to the bottom of the canyon and pinning his arm against the canyon wall, trapping him. He initially yells for help, but nobody is within earshot. As he resigns himself to the fact that he is on his own, he begins recording a video diary on his camera and using his pocket multi-tool to attempt to chip away at the boulder. He also begins rationing his water and food.
As he realizes his efforts to chip away at the boulder are futile, he begins to attempt to cut into his arm, but finds his knife too dull to break his skin. He then stabs his arm, but realizes he will not be able to cut through the bone. He finds himself out of water and is forced to drink his own urine. His video logs become more and more desperate as he becomes delusional, dreaming about relationships and past experiences, including a former lover (Clémence Poésy), family (Lizzy Caplan, Treat Williams, Kate Burton), and the two hikers he met before his accident. After reflecting upon his life, he comes to the realization that everything he has done has led him to this ordeal, and that he was destined to die alone in the canyon.
After five days, a delusional Ralston sees his unborn son through a premonition. He gathers the will to apply enough force to his forearm to break it and severs his arm with the dull knife, fashioning a crude tourniquet out of the insulation for his CamelBak tube and using a carabiner to tighten it. He wraps the stump of his arm and takes a picture of the severed portion he is leaving behind. He then makes his way out of the canyon, where he is forced to rappel down a 65-foot rock face and hike several miles before, exhausted and covered in blood, he finally runs into a family on a day hike. The family sends for help and Ralston is evacuated by helicopter.
The film ends with shots of the real Aron Ralston from his life after his ordeal — including several of Ralston's further adventures in climbing and mountaineering, which he continued following the accident — and of Ralston with his wife, whom he met years later, and their son.