NOTE: This spoiler was sent in by EastofPgh.
The movie starts off following three friends, (Sarah, Juno, and Beth,) on a white water rafting trip. They're going down the rapids as a little girl and a man stand and watch from downstream. The little girl and man turn out to be Sarah's husband and Daughter. The three girls dock on the small beach there, and get out of the raft. Sarah and Beth are moving equipment, while Sarah's husband helps Juno out of the raft, and takes off her helmet Beth notices a weird moment pass between the two (insinuating that Juno and Sarah's husband were having an affair,) but doesn't say anything to Sarah.
Beth suggests Sarah returns to the hotel early with her husband and daughter, and she and Juno would get the raft and equipment. Sarah agrees and it changes scenes to inside Sarah's car. She's talking to her daughter in the back seat first, about her upcoming birthday and who they could invite. Then she starts talking to her husband, who takes his eyes off the road. The car drifts over into the other lane, and crashes into another car, that has a bunch of poles on the top. One pole slides right though the windshield, and right through the husband's head, and into the daughter, killing them both while Sarah gets knocked out.
There's a scene in the hospital, where Sarah runs down the hallway, as the lights behind her go out. It ends rather abruptly, as Beth grabs her and both of them fall to the floor, Sarah sobbing, and Beth telling her that her husband and daughter were both gone. That scene was sort of the beginning of Sarah's 'descent'.
It skips one year ahead, and to the Appalachian Mountains. Sarah and Beth are heading towards a cabin Juno and another group of friends is staying at, in anticipation of a cave exploration the next day. They get there, and we're introduced to the other characters:
Holly: A rugged outdoorsy girl. Seems a bit of a lesbian. She's kind of abrasive and hates following directions. Most experienced cave explorer.
Sam: Older sister to Becca.
Becca: Sam's younger sister. Doctor in training.
All the girls have a night of memory swapping, during which Sarah seems a bit distant. She eventually says during the conversation: "Love Each Day." and explains it as something her husband used to say. Ends up being important later. That night Sarah wakes up, hears a child's laugh, heads over to the window, just in time for a metal pole to break through the window and go right through her head. Turns out to be a dream and another sign of her Descent.
The next day the girls are headed towards what they think are the 'Borem Caves'. Holly's a little bit moody, because according to the guide book, the Borem caves are for beginners, and she's afraid she won't have fun. The girls eventually park the car, and get their gear and get ready to hike through the woods a bit. Juno leaves the guide book to the Borem Caves behind.
They eventually get to the mouth of the cave, which is a hole in the ground, that they can't see the bottom of. They repel down one at a time, and are in awe of their surroundings. It's pretty neat, and mystical for a moment. Sarah ends up waking up a bunch of bats though, that swarm around her for a second before heading up out of the hole. She gets calmed down by Beth. Juno gets them all ready to move again, and tells them, "The only way out, is down the pipe," which turns out to be an extremely small hole that they must crawl through, one at a time.
They get through it, and enter a huge cavern, where they light a flare, and stop for lunch. Sarah doesn't feel like eating, and ends up walking around for a few minutes, away from the group. She hears a child's laughter again, just like in her dream, and turns her head to try and find the source of the noise, before Juno walks up behind her and startles her. Sarah gets back to walking around, and finds the next tunnel, another small passage they must crawl through one at a time.
All the girls go through, and Sarah's going last. Beth is in front of her. Around the last bend in the tunnel, Sarah ends up wedging herself in at an angle that she can no longer move. She screams that she's stuck, and starts to panic. Beth crawls back down to her, and asks her: "Why are you panicking? What are you so afraid of?" But Sarah won't say. So Beth calms her down by telling her a joke, and gets Sarah to move in a way that she's no longer stuck. Sarah remembers that she forgot the rope bag behind her in the tunnel, and starts to move backwards to retrieve it, when the stone directly above her slides down an inch or two. Both girls pause, and stop breathing for a second, before Beth screams *beep* the rope bag, move! Now!" and the girls quickly crawl up the rest of the tunnel as the stones collapse behind them, effectively caving them in.
In the next chamber, the girls are panicking. Sam says not to worry, she put out a notice that they would be going to Borem Caves. Juno avoids the subject. Someone points out that something's not right. The caves are nothing like what the guide book said they'd be like. They turn to Juno for the guide book, at which point she reveals that she left it behind, because they were NOT at Borem caves at all. She had discovered this cave system herself, and wanted her and her friends to be the first to explore it. The girls are, as expected, pissed off at her for lying to them.
There's a scene a bit further ahead in the movie that we finally get to see the tension between Sarah and Juno in. The girls must repel across a cavern, (because there's no floor) and Sarah and Juno are the last ones left on the first ledge. Juno says to Sarah: "I thought we could name the caves after you." To which Sarah says back: "How about after you?" In an extremely condescending way, which makes it clear that Sarah does NOT trust Juno, and does not like her. We have to assume that she probably knows about the Affair.
Later in the movie, Juno is using Holly's lighter to find the correct path to take. Flame reacts to oxygen, so when at the right tunnel, the flame flickers. When they find the right one, Holly gets excited, and runs ahead. Juno shouts for her to slow down, it's too dangerous to run ahead, and quickly follows her. Holly's shouting that she sees daylight, and Juno screams that "It's not daylight!" (It was explained as sulfur excretions off the rocks or something to that effect) But it's too late. Holly falls right into this hole. As she's falling down it, Juno jumps on her stomach and grabs her arm.
Holly's begging her to pull her back up, but Juno can't muster the strength to, and Holly falls down the hole, into the shadows pretty roughly. The girls repel down after her, and find out that Holly's broken her leg.
The girls move her out of the water that she's fallen in, and to dry rock and start to work on her leg, Becca shouting instructions, while Sarah wanders off. She heads down this passage with a flashlight. The passage opens up into this cavern where water covers the floor. Sarah hears something move, out in the center of the room. Quickly, she shines her light in the center of the room, searching for the source of the sound. When finally, she finds it. A crouching, extremely pale figure, taking something out of the water and biting at it. It looks like a man, but way skinnier. Sarah gasps out of fear, and the creature turns its head and looks directly at her, before Juno grabs Sarah's shoulder and asks her what she thinks she's doing, wandering off by herself. Sarah tells her she saw someone, and shines her light back in the middle of the room, but the creature is gone.
The girls get Holly's leg bandaged, and continue on. They come to a room where the wall in front of them rises up a bit, and they have to climb it. They help get Holly up, then the rest of them climb up. It's obvious that not all is well here. There are bones covering the floor. One of the girls, (Beth I think) asks for the video camera. She turns on the infrared, and starts looking around through the viewfinder. She says: "Bones. Thousands of them. This isn't right." and then Sam, fueled by Sarah's earlier sighting of someone who looked like a man, starts shouting: "Hello??? Is there anybody there?" Beth spins around with the camera, and standing there, right behind Sam and Becca, is one of the creatures, (dubbed Crawlers by the viewers on IMDB,). It was probably the most frightening moment I've ever seen in a movie. The thing lets out the WORST screech/roar ever, and all the girls start screaming before it 'crawls' away on all fours like a wild animal, and starts crawling up the walls.
All the girls are screaming, and freaking out, saying that that was no man, and that they needed to get out of there. There are like 3 ways they could go. They start towards one, but the Creature screeches from above them, like it's following them. Eventually, all the girls break up and run away from each other, and then down one path. Holly's sort of been left behind by everyone except for Juno, who tries to help her. But she's pretty much too late. The Crawler jumps on Holly, and rips her throat out... With its teeth. Juno goes into attack mode and starts fighting the Crawler, just like Buffy would, except not all martial artsy.
Juno eventually kills it with her rock climbing Pick. But all the trouble is not over yet. Beth walks up behind Juno, terrified, and traumatized, and doesn't even warn the girl. Juno hears her feet on the rock behind her, and spins around, attacking without looking, and her Pick rips right through the back of Beth's neck, right through her throat. Juno is completely shocked, and does the only thing that makes sense to her at the time, and backs away, and runs off down the passage way, as Beth lays on the ground in shock, clutching at her throat.
Meanwhile, Sam and Becca have run one way, while Sarah ran another. Sarah falls down this short drop, and bumps her head. Her helmet light goes out.
She wakes up a short time later, inside this seemingly endless cavern of bones. Human, animal, you name it. She is just about to move, when Holly's body is dropped from a hole in the ceiling, and the Crawlers swarm in after it, and tear into it, in a pretty gory scene. Sarah takes out the video camera and tapes them doing it, so she can get a good look at them, (look for weaknesses). The sight of what they're doing to Holly makes her sick, and she starts to throw up, when a Crawler turns and looks right at her. Sarah shuts right up, and we get an up close and personal look at the Crawler's face through the video screen. We see that the Crawlers are in fact, BLIND. So being quiet will keep you safe.
Becca and Sam are tormented in a few scenes by Crawlers, but they too discover they're blind and stay safe, until Juno, who herself has been lost, alone in the caves, finds them and rescues them from a Crawler. She tells them she's found the way out, but they're not leaving without Sarah.
Meanwhile, Sarah heads down that cavern. She finds Beth, who is STILL alive, but barely. Beth gives Sarah Juno's bracelet, which she dropped after stabbing Beth. The bracelet says "Love Each Day" on it. The phrase Sarah's husband used to say. Beth tells Sarah not to trust Juno, and then begs Sarah to kill her. Sarah crushes her skull with a rock. You see it from a distance, so you don't see gore. Sarah uses her supplies to create a torch, and heads on alone.
Back with the other girls, after being chased by a swarm of crawlers, they come to another room where they have to repel to another ledge. Becca goes on ahead, and gets to the middle of the ceiling, when she notices a Crawler in the room. She takes out a knife, and puts it between her teeth, holding it for later, as the crawler moves towards her. The two fight for a second, Becca hanging from the rope, before the crawler slits her throat with its fingernails. In her dying moments, Becca hops on its back, and slits its own throat with her knife, before falling backwards and dying, hanging from the rope.
Sam starts screaming, and is grabbed by another Crawler coming through the small tunnel behind her. It pulls her down. She flips onto her back to try and kick it away, but it overpowers her and rips into her stomach, and starts ripping her intestines out with its teeth, all while she's still alive. Juno tries to pull her back, but they end up dragging Sam's body away.
Sarah has her own fights with the Crawlers, and kills two of them, 1 a child, and the other its mother, (The only female crawler seen in the film). After she kills them, she goes completely wacko, and starts bellowing, at the top of her lungs. More Descent into madness there. People theorize that the more savage she became, the more like the Crawlers she became.
Juno ends up catching up to Sarah eventually, and leads her to the way out. Oh, I forgot to explain, the girls stumbled onto a cave painting, which showed the mountain, and two caves leading from it, which shows that there was in fact two exits from it. It also insinuates that the Crawlers are descended from Cavemen, who just decided to stay inside the caves instead of heading into sunlight.
Near the exit, a swarm of about 5 Crawlers attack, and there's a HUGE fight scene. Both the girls kill all of the Crawlers, and stand up to face each other. Sarah's looking at Juno with utter HATE in her eyes, and the Pick in her hand. Juno looks surprised for a minute, before Sarah allows the Bracelet to drop into view from her closed fist, showing that Sarah now has proof of what Juno's done, and she's not havin' any of that. In one final act of betrayal, Sarah swings downwards with the Pick, and breaks Juno's leg with it, and leaves her behind for another swarm of Crawlers to overtake her.
Sarah crawls up this mountainous pile of skeletons, and exits the caves screaming, and bathing in the sunlight. She runs back to the cars, and jumps in hers, and drives off as quickly as she can. She's literally speeding her way out of the forest.
The US Ending:
When she gets to Highway, she pulls over and rolls down her window, and throws up outside. When she sits back up, Ghost! Juno, looking just like a Crawler, is sitting next to her. Sarah screams.
The UK Ending:
Right after seeing Juno's ghost and screaming, Sarah awakens again. She is *back*, inside the Cave again. She has a vision of her daughter, sitting in front of her with a birthday cake, and Sarah stares at her, smiling a little. The camera pulls away, and we see that there is no daughter. Sarah is staring at nothing. While the Crawlers are screeching and roaring in the darkness in the background.
My take on the ending: The events in the cave finalized Sarah's "Descent" into madness. She was driven completely insane by what she saw in the caves, and it ended up that the vision of her daughter was where she was comfortable. She allowed herself to believe that that was real for her, and that nothing else was. So basically, Sarah never makes it out. And probably none of the other girls do either.