Aron Ralston (James Franco) goes hiking and climbing in a canyon in Utah. After acting as a guide for two fellow hikers (Amber Tamblyn and Kate Mara), Aron falls after a boulder comes lose and he ends up trapped when the boulder crushes his hand against the wall of a cavern. Throughout the next five days, Aron tries to chip away at the boulder with a knife, while having many different flashbacks and hallucinations about his life. With his water and food running low, Aron regrets not telling anyone where he was going, since he figured that he would be dead before anyone started looking for him.
Aron tries on three occasions to cut his arm from the rock. One the first try, his knife was too dull and barely scratched the flesh. On his second try, he successfully stabs through his arm, but stops when he feels the bone. On his third try, Aron is finally successful in amputating his arm after he snaps his bone in half against the boulder and then (slowly) cuts off his arm with a small penknife.
After he is free, he creates a sling for his arm with his bag and leaves the cavern. He manages to climb down the mountain and he is saved when he is spotted by a family of hikers, who call for help.
The final shot of the film features Franco's Ralson seeing a vision of the real Aron Ralston sitting with his wife and new kid. Titles say that Aron Ralston still goes mountain climbing, though he now makes sure to leave a note saying where he's going.