For roughly half of its nearly two-hour-long runtime, the infamously disturbing backwoods drama “Deliverance” very convincingly pretends to be something it isn’t. If you were to start the film with no knowledge of plot or tone (given the sheer extent of the film’s notoriety, that would admittedly be difficult), you’d believe it’s nothing more than a light, easygoing buddy comedy about four friends canoeing through northern Georgia’s towering mountains, if only to catch one last glimpse of this untamed wilderness before all those waters are dammed.

These friends are Ed Gentry, Lewis Medlock, Bobby Trippe, and Drew Ballinger, all (thanks to Lewis) willing to find what lies just down the Cahulawassee River. Though these raging rapids and empty woods aren’t exactly what they’re used to, they’re nevertheless prepared for any trouble that might come their way. As they soon come to realize, however, that’s only how it seems.

Even before uncertainty and danger begin creeping in, “Deliverance” offers a portrait of the great outdoors that is hardly bright or colorful; but as the tone sours, it turns downright menacing. Emphasizing this, very little score is ever-present except in the midst of gripping, shockingly violent terror. Taken in hindsight, even the rousing “Dueling Banjos” musical number opening the film feels more like a slap in the face.

With any remote semblance of control slipping further and further from the characters’ fingers, “Deliverance” impressively sustains the oppressive dread looming over their heads — thanks to how vividly the grainy, claustrophobic cinematography and atmospheric sound design bring the surrounding woods to life. If anything, these perilous wilds are the real main character here, even with the entire cast’s performances being uniformly excellent.

With all this in mind, it’s easy to see why “Deliverance” is so highly regarded as a genre classic, to the point of being selected for preservation in the United States National Film Registry for being “culturally, historically, or aesthetically significant.” In my eyes, however, it unfortunately falls just short of being a genuine masterpiece simply because of how abruptly this twisted, treacherous journey ends.

Nothing about this conclusion feels cheated or poorly thought-out, but with so much tense, heart-pounding buildup throughout this story, this sudden screech to a halt is rather frustrating. Considering the terrible situation these characters are caught up in, so much more could’ve perhaps been explored by this point.

As the brutal consequences of their panicked decisions rapidly snowball into worse and worse decisions, it’s clear this eerie suspense should have been stretched out much longer. At the very least, just 20 additional minutes of rising unease wouldn’t have felt out of place, nor prevented the story’s ultimate resolution from holding together.

Regardless, this is undoubtedly among the most unique survival thrillers out there, even if it may very well ruin your night. That could be a turnoff for some, but when telling such an ugly, uncomfortable story, it’s better not to sugarcoat what takes place. Still, even if you’re up for watching it once, you might never find yourself eager to see it ever again.