A whitewater rafting trip is underway, with 3 long-time friends on board. Leading the trip is the bold adventurer, Juno (Natalie Mendoza
). Also there is wife and mother Sarah (Shauna Macdonald
) and her closest friend, Beth (Alex Reid
), both British. Awaiting at the end of the raft adventure is Sarah's husband, Paul (Oliver Milburn
) and daughter, Jessica (Molly Kayll
). Paul and Juno share a strange moment before he drives to go back home with Sarah and Jessica. While Sarah and Paul are seemingly on the cusp of an argument, a small truck plows head-on to their car, sending a metal rod right through Paul's skull.
Sarah has the first in a series of dreams where Jessica appears in a party dress and seems to offer Sarah a birthday cake. Awakening from her injuries in the accident in the hospital, Sarah panics and runs down the hallway, imagining the hospital getting dark around her. Beth embraces her and they both weep. Juno, standing in the distance and also crying, turns and leaves them.
A year later, Juno has organized a spelunking (caving-diving) trip in the Appalachian Mountains. The three women from the raft trip gather at a cabin the day before this adventure. An additional 3 adventurers join them, including Juno's butch friend Holly (Nora-Jane Noone
), who even Juno considers too reckless. The other two are a pair of European-rooted sisters, the safety-cautious, older sister, Rebecca (Saskia Mulder
) and medical student Sam (MyAnna Buring
), who, though competent, is the least experienced adventurer of the group.Tension remains between Juno and Beth and Sarah over Juno's abandonment of them last year. Holly laments the the cave they are to explore is just a tourist trap and offers no challenges. That night, Sarah has a nightmare that a steel rod hits her face just as it did Paul's.
Waking her five companions the following morning, Juno leads them on a long drive over rough country road. When leaving the cars, Juno seems to intentionally leave behind the cave map. Hiking to the cave entrance, they encounter the huge carcass of a bull elk, with his throat seemingly having been torn open. All the women except Juno and Holly become hesitant when they see the cavernous entrance to the cave. They all rappel down into the first pocket in the cave. Descending still further, all daylight is vanished. Juno lights a series of flares, illuminating the huge cavern of the cave system that they've entered. Even Holly is impressed, recording her surroundings with her high-tech night-vision digital camera. As the women temporarily further prepare, Juno indicates they are looking for a tunnel to bring them further into the cave system. Juno apologizes to Sarah for leaving the previous year and grows tearful when recalling Paul. Seemingly trying to avoid this conversation, Sarah goes off on her own and finds a small tunnel entrance.
One by one, the women go into the tunnel, which grows even smaller midway before coming out in another large space of the caves. Sarah is the last to come through and gets stuck on a rock. Sarah turns to claustrophobic panic. Beth, at first, tries to calm Sarah and then climbs back down the tunnel to help her out. As Beth pulls Sarah out, the tunnel roof begins to tremble and, just as the two pull out, totally collapses behind them. The whole group is distraught at the tunnel collapse, since it is obviously their return route out of the caves. Juno then reveals to the others that she not only didn't bring the cave map, but she has intentionally taken them to an unexplored cave rather than the cave to which she claimed she was taking them. Juno desired to be the first to explore these caves and have the caves named after her. The others are angered, but Juno informs them that they must keep moving and look for another exit to the cave lest they die in there.
The group comes to an enormous precipice in the cave. In order to get across to a hopeful exit, they must set up a pulley system on the roof of the precipice. Rebecca painfully climbs along the roof using only her arms, hooking the rope to the cave roof. Finally arriving to the opposite side of the precipice, Rebecca sees a hook obviously left by a previous explorer of the cave. Each of the women cross the precipice using the pulley system and Juno goes last. Claiming they will need all the equipment they can get, Juno pulls down the rope pulley as she crosses the roof. Near the end, Juno loses her grip and falls. Rebecca must grab the rope to get keep Juno from falling and her hands nearly tear in half supporting Juno's weight. Juno finally climbs up the precipice wall to join the others.
As the six women move on, a drooling mouth is seen in the foreground but is not detected by the women. When walking through the caves, they encounter an ancient-looking mural, resembling a community drawn by early Native Americans. Beth notices that a second exit from the cave is shown on the mural.
When it's time to continue, Holly runs in the front. She sees what appears to be daylight (but is actually just phosphorescent rock) and rushes even more, despite Juno telling her to slow down. Suddenly Holly falls down a huge pit in the cave. Her leg is snapped in the fall, her bone painfully protruding from her skin. All the others rappel down into the pit and tend to Holly. Sam pulls Holly's leg back into place and makes a makeshift tourniquet for her. Meanwhile, Sarah has wandered away from the group. She finds an old, rusty miner's helmet. Looking up she is shocked to see a grotesque creature in the distance. The creature appears humanoid, but is hairless and deformed-looking with sharp claws and small eye sockets. In a moment, the creature disappears as Juno comes to find Sarah. Juno and the group don't believe Sarah about the creature and they all, including the limping Holly, go on to find a way out of the cave.
They come to a part of the cave with three tunnels and even Juno is vexed as to which one to go down. A blood-curdling noise is heard, frightening the women. Suddenly the grotesque creature Sarah had seen is standing next to the women. Within seconds, the creature climbs with startling speed up the cave wall and out of sight. The creature then jumps on top of Holly. The group is frightened and Sam and Rebecca run down one of the tunnels and Sarah another. Sarah shortly loses her footing while running and falls down another pit, knocking her unconscious.
The creature bites Holly in the throat, killing her. Juno begins to get into a tug-of-war with the hideous beast over Holly's body. When the creature confronts Juno she slashes its face with her pick-axe. Another similar creature jumps onto Juno. The injured creature drags off Holly's body as the second creature struggles with Juno. Juno finally kills the second creature with her pick-axe. Seconds after killing the creature, Juno hears something coming up behind her and swings the pick-axe. It is actually Beth and Juno has stabbed her through the throat. Grabbing Juno's bracelet, Beth collapses in a pool of her own blood and pleads with Juno to help her. A shocked Juno backs away into one of the tunnels, leaving Beth to die.
Sarah has another dream about her daughter and the cake, but her daughter now has the face of a creature. She awakes and realizes she's in a dark pit of fresh kills, practically flooded with the blood and entrails of various animals. Suddenly, the body of Holly is dropped into the pit. Using Holly's night vision camera to see, Sarah watches in horror as several of the creatures, yelping like hyenas and making clicking noises like the echolocation of bats, ravenously devour Holly. Gagging at the sight, Sarah causes one of the creatures to become aware of her presence. Coming to within inches of Sarah's face, it seems unable to detect her presence as she doesn't move a muscle. Juno is soon heard above calling from the others. The creatures all descend from the feeding pit to go after the other prey. Sarah makes a torch so she can find her way out of the pitch-black feeding pit.
One of the creatures happens into Sam and Rebecca, who are embracing and remaining as still as possible. This creature roars in their general direction, but fails to see them, confirming that the creatures are totally blind and instead rely on sound to hunt. Juno again calls out for the others and the creature leaves Sam and Rebecca's vicinity. Sam and Rebecca move on to hopefully find a way out of the caves. Turning a corner, they are attacked by one of the creatures. The creature tries to maul Rebecca while Sam is frozen with fear. However, Juno appears and stabs this creature through the skull with her pick-axe, killing it.
Meanwhile, Sarah is also trying to escape, returning to where they came in and finds Beth. Beth has lost a lot of blood and explains to Sarah that Juno had stabbed her and left her. Beth shows Sarah Juno's bracelet as proof. Beth pleads with Sarah to mercy-kill her, which a mournful Sarah does with a nearby rock. Moments later, a small, child-like creature appears and attacks Sarah, jumping on her back. Sarah smashes the small creature into the cave roof and flips it onto the floor, killing it.
Within moments, another creature appears, this one apparently a female with long hair and distinct breasts and also apparently the small creature's mother. When the female creature attacks her, Sarah knocks her back and then she falls back into another pit. This time she falls into what appears to be a pond of blood. As she tries to climb out of the blood pond, Sarah is again attacked by the female creature. Though the creature has her pinned down, Sarah manages to reach a nearby jagged bone and stabs the female creature through the face, killing her. A male creature then arrives, failing to detect Sarah's presence as she lies motionless beneath his feet. Sarah assaults the creature, managing to kill him also with the jagged bone. Traumatized and bloodied, Sarah lets out a blood-curdling scream.
Juno, Rebecca, and Sam hear Sarah's scream. Juno vows not to leave the caves without Sarah, but changes her tune when she turns to see about a dozen of the creatures crawling in pursuit of them. The three of them run off down the tunnel. Sam takes the lead and comes to another precipice. Though she knows she is too short on rope, she tries to establish another rope-pulley system to get across. When Juno and Rebecca run up behind, they plead with Sam to be careful. A creature jumps out, clings to the cave roof, and attacks Sam. It manages to rip out Sam's throat, but Sam, with her last ounce of strength, stabs the creature, which falls into the water at the bottom. The pursuing creatures arrive and pull down Rebecca. They rip open the screaming Rebecca's stomach and throat, killing her. Cornered at the edge of the precipice, Juno jumps down.
Juno lands in a large pool of water at the bottom. Underwater, the creature awaits Juno, but she uses Rebecca's knife to kill it. Juno climbs up the slippery walls, but she loses her grip and begins to fall again. Suddenly, a hand grabs her, pulling her up. It turns out to be Sarah. Sarah is now bloodied and nearly silent, looking upon Juno with poorly-disguised scorn for harming Beth. Juno and Sarah run off, still looking for an escape. Coming into a cavern, they realized they are surrounded by four of the creatures. When the creatures close in, a violent struggle ensues. Juno and Sarah each kill a creature with their pick-axes, but are disarmed by a second wave of attacking creatures. Sarah kills her attacker by gouging out his eyes, while Juno kills the other by stabbing him through the throat with Sarah's flaming torch.
After the fight, a tense stand-off with the pick-axes occurs between Juno and Sarah. Momentarily distracted by the sound of more creatures coming near, Juno is stabbed through the leg by Sarah's pick-axe. Just as Juno left Beth to die, Sarah leaves Juno to die. As she runs off, Sarah hears the sounds of the creatures attacking Juno and Juno screaming.
Sarah runs into a side cavern and finds it full of bones. At the top of the cavern, daylight peeks through. Sarah frantically climbs to the light and pulls herself up into the woods above, blinded by the daylight. Now maniacal, Sarah runs back to the cars and takes off in one of them. Driving off back down the country roads they had come in on, Sarah is startled by the bloodied figure of Juno appearing in the passenger seat. Shifting her head, Sarah realizes that she's still in the cavern and that her escape was a fantasy. The cavern actually is a dead-end. To comfort herself, Sarah fantasizes about her daughter Jessica giving her the birthday cake again. As Sarah drifts off in her own mind, the sounds of the creatures draw nearer.