Forget ‘Saw 3-D.’ This is authentic horror.

This Halloween weekend the blood will be flying on screens across the land as one of horror’s goriest franchises springs back to life with Saw 3-D. But if you’re looking for a truly horrifying, and more authentic, tale of amputation, wait for 127 Hours, which opens in Canada Nov. 12. In this harrowing true story, that classic horror trope—don’t go down to the basement!—takes on a whole new twist. The “basement” is the bottom of a slot canyon in a remote Utah desert, and the bogeyman is a “chockstone” boulder that falls on a climber and traps him there for five days—until he finally frees himself by sawing through his right forearm with a cheap stocking-stuffer penknife.

Based on the 2004 memoir by Aron Ralston, 127 Hours is the latest film from Danny Boyle, who dramatizes Ralston’s story with a gonzo assault on the senses that makes the director’s previous films, from Trainspotting to Slumdog Millionaire, feel tame by comparison. For a movie that consists largely of one actor agonizingly stuck in the same place, there’s nothing static about 127 Hours. And considering that we know how the story ends before we go in (“Two tickets please for the movie about the guy who cuts his arm off”), it’s remarkably suspenseful—a roller-coaster ride that draws gasps and shrieks, and left this critic emotionally and physically exhausted. During the film’s premiere last month at the Toronto International Film Festival, the climactic scene of Ralston (James Franco) performing surgery on himself had some viewers feeling faint and bolting for the exits.

In a Maclean’s interview after the premiere, Boyle—a gregarious Brit whose charm embodies the kinetic energy of his filmmaking—heartily defended his graphic rendering of the story. He said, “The only way you’ll ever be able to watch it, and bear it—and not everybody can—is if you feel like you’re complicit in it. If it’s a first-person experience that’s immersive and you are helping Aron do it: Go on! Go! . . . Oh no!” Boyle felt it was essential for the audience to appreciate the visceral details of Ralston’s ordeal. “The most extraordinary thing about the arm,” he said, “is that it took him 44 minutes, 44 minutes with a blunt knife. It’s typical of him, timing everything, trying to control everything to keep his sanity.” Anyone who finds the movie too graphic should read the book’s five-page description of severing muscle fibres and “guitar-string” tendons one by one.

Ralston, 35, who has reinvented himself as a mountain guide and motivational speaker, worked as a consultant on the film. “The first time I met him,” Boyle recalled, “I said, ‘Why didn’t you cut through the elbow?’ ” Of course, anyone who’s carved a turkey would think of that. But instead Ralston broke both bones in his forearm before he commenced surgery. Going through the elbow, he told Boyle, “never even crossed my mind.”

But a lot of other things did as he contemplated a death he felt was inescapable. While pinned by the boulder, with his one free hand, he taped his final thoughts for his family on a video camera—a useful device in the movie. It gives James Franco something to talk to, like the volleyball that co-stars with Tom Hanks in Cast Away. The movie also takes us into Ralston’s fevered hallucinations, including the vision that motivated him to get free. “I saw myself have an out-of-body experience,” he told Maclean’s, “where I walked through the wall of the canyon, down a hallway, and through a doorway into a living room where I saw this little blond, blue-eyed boy playing with a truck on a sunlit floor. He drops the truck, comes over to me, and I bend down, scoop him up with my handless right arm and put him on my shoulder. There’s a wordless exchange and he says, ‘Dad, I’m so glad to see you. Can we play now?’ Then flash, it’s gone. But it told me that was my future son.”

Ralston, who married in 2009, now has a baby boy, coincidentally with blond hair and blue eyes—unlikely features given he and his wife have neither. He still climbs. “But if I get in a spot where it’s a little sketchy,” he says, “previously it would be, ‘Take a breath and it’s going to be okay.’ Now it’s: ‘Take a breath and think about the baby.’ ”