The 127 hours in question entail the 5 days that Ralston spent in a crevice in Utah's Blue John Canyon. He had been climbing on his own and hadn't told anyone where he was going. When he falls and finds his arm trapped under a boulder, he has to make a terrrible choice in order to survive.
WARNING: SPOILERS AHEAD
The trick to making a one-man show - which this essentially is - work is to go into his mind, which the film does. Unsurprisingly, after a few days of being trapped with dwindling supplies of food and drink, Ralston took to a bit of light hallucinating.
Before this, though, Ralston's encounter with two young women who are lost in the canyon, show us what he once was: spontaneous, looking for fun, self-sufficient. There are some typically unusual Danny Boyle shots at the start, like showing us the inside of the straw to Ralston's water bottle as he sucks the liquid out, and these Innerspace-style moments become a shorthand later, as he faces the possibility of death.
Foreshadowing, in a light-hearted way, his own later wrestle with fear, he persuades the girls to take the scenic route to their destination, and gets them to crawl into a narrow corridor of rock, and then drop into a latge, deep pool below. As with most thing, the first time is the worst, and soon they're all going again, and capturing the almighty splashes on Ralston's camcorder.
Ralston is obsessive about documenting his journey - especially for someone who seems to so relish his solitude. Certain frames are seen once normally and then once again through a camera or camcorder screen. For a story that's set in 2003, it's prescient of the way we live now. The real Ralston documented his time trapped by the boulder on his camcorder, but only those very close to him - he imagined speaking to his family and friends as he recorded himself - have seen the footage.
Most of the film takes place in the tiny corridor that Ralston falls into. At first, he frantically chips away at the stone pinning his arm to the wall with a blunted penknife, but, over time, he comes to accept that the only way out is to leave part of himself behind. As hunger, dehydration and the pure horror of his situation sink in, Ralston imagines his family and friends, records what he believes to be his last words and even etches his presumed death date into the wall.
Franco's performance is very convincing, and the cinematography flips between showing us the tiny inner workings of Ralston's body - the yellow liquid disappearing from the water bottle as he's reduced to drinking his own urine, the blunt knife hitting his arm bone as he works out how to sever it - and the vastness of the unhearing world outside. The camera tracks out from Ralston's narrow little prison to show us the vast moonscape of the canyon, where the only other living creatures are birds. Ralston really doesn't have a choice.
And so to the amputation scene, which has be known to cause the light headedness and the fainting. From the breaking of the bones to the poking about with the knife, it's very graphic and difficult to watch. Try doing so from behind interlaced fingers - it's just the bottom right hand corner of the screen that you'll be wanting to avoid, but you don't want to miss the rest of it.