In April 2003, avid mountaineer Aron Ralston goes hiking at Utah's Canyonlands National Park without telling anyone. He befriends lost hikers Kristi and Megan and gives them directions. While guiding them, he shows them an underground pool. The three have a good time there, and that afternoon, the girls invite him to a party that is being held the following night. Aron accepts and says goodbye to the girls, who head home. He continues through a slot canyon in Bluejohn Canyon. While climbing, an 800 lb (362kg) boulder he is hanging off comes loose and causes both to fall, which traps his right arm against the wall. Aron attempts to move the boulder, but it does not budge; he also soon realizes he is alone. He shortly begins recording a video diary using his camcorder to maintain morale as he chips away parts of the boulder with a pocket knife. At one point, the knife falls off his hand, and he is forced to use his bare feet and a little branch to recover it. Over the next five days, Aron rations his food and remaining 300ml of water, struggles to keep warm at night, and is forced to drink his urine when his water runs out. He also sets up a pulley using his climbing rope in a futile attempt to lift the boulder.
Throughout the days, Aron becomes desperate and depressed and begins hallucinating about escape, relationships, and past experiences, including his family and his former girlfriend, Rana. He also imagines going to the party he was invited to and having fun. During one hallucination, Aron realizes his mistake was that he did not tell anyone where he was going or for how long. Aron has a vision of his future son on the sixth day, spurring his will to survive. He fashions a tourniquet from CamelBak tube insulation and uses a carabiner to tighten it. Then, using his knowledge of torque, he breaks the bones in his arm and, using the multi-tool, slowly amputates it. Aron then wraps the stump to prevent exsanguination and takes a picture of the boulder before rappelling down a 65 ft (20 m) rockface. He then finds some rainwater collected while descending down, drinks the stagnant water due to dehydration, and continues. He spots a family on a hike in the desert and calls for help. They give him water and alert the authorities; a Utah Highway Patrol helicopter brings him to a hospital.
A textual epilogue reveals that years later, Aron got married and had a son. He also continues climbing and always leaves a note telling his family where he has gone.
