Descent
On her way back from whitewater rafting with her friends Juno (Natalie Mendoza) and Beth (Alex Reid), Sarah (Shauna Macdonald), along with her husband Paul (Oliver Milburn) and their daughter Jessica (Molly Kayll) get involved in a car accident when Paul is distracted. Paul and Jessica are killed, but Sarah survives.
One year later, Sarah and her friends Juno, Beth, Sam (MyAnna Buring), Rebecca (Saskia Mulder), and newcomer Holly (Nora-Jane Noone) are reunited at a cabin in the Appalachian Mountains of North Carolina for a caving adventure. The next day, they hike up to a cave entrance and descend. While in the cave, Juno apologises to Sarah for not being there for her after the accident, but Sarah is distant. As the group moves through a narrow passage, it collapses behind them, trapping them. After a heated discussion, Juno admits she has led the group into an unknown cave system instead of the fully explored cave system that they had originally planned, and that rescue is impossible because of this. She then tells Sarah that she led them into the unknown cave in the hopes of restoring their relationship, but Sarah rebuffs her.
As the group presses forward in the hopes of finding an exit, they discover climbing equipment from a previous caver, and a cave painting that suggests an exit exists. Holly, thinking she sees sunlight, runs ahead, but falls down a hole and breaks her leg. As the others help Holly, Sarah wanders off and observes a pale, humanoid creature drinking at a pool before it scampers away. Later, the group comes across a den of animal bones, and are suddenly attacked by the creature (called "crawlers" in the credits). Holly is killed, her neck ripped out before being dragged away, while Sarah runs and is knocked out by a fall down a hole. Juno, trying to prevent Holly's body from being dragged away, kills a crawler with her pickaxe before accidentally stabbing Beth through the neck when she is startled. Beth collapses with Juno's pendant, and a traumatized Juno flees while Beth begs her not to leave her.
Sarah awakens to find herself in a den of human and animal carcasses, and witnesses Holly's body being eaten by a group of crawlers. Juno discovers markings pointing to a specific path through the caves. After regrouping with Sam and Rebecca, and realizing the crawlers' use of sound to hunt, Juno tells them the markings she discovered may point to the way out, but she will not leave without Sarah. Meanwhile, Sarah encounters Beth, who tells Sarah that Juno wounded her and abandoned her. Beth also gives her Juno's pendant which Sarah recognizes as a gift from Paul, realizing that Juno had an affair with Paul before his death. In pain and not wanting to be eaten by the crawlers, Beth begs Sarah to euthanize her, Sarah reluctantly does so by bashing her head in with a rock. She then encounters a family of crawlers, but manages to kill them all, being covered in blood in the process. Afterwards, she has a psychotic breakdown. Elsewhere, Juno, Sam, and Rebecca are pursued by a large group of crawlers. When they reach a chasm, Sam tries to climb across but encounters a crawler scaling the ceiling. It rips her throat out but Sam stabs it before she bleeds to death in front of Juno and Rebecca. Rebecca is then dragged back and eaten alive as Juno escapes.
Juno encounters Sarah and lies to her about seeing Beth die. After defeating a group of crawlers, Sarah confronts Juno, revealing that she knows that Juno had wounded Beth and left her for dead, and also about the affair. She then strikes Juno in the leg with a pickaxe and leaves her to die to a swarm of approaching crawlers. Juno is last heard screaming as Sarah escapes. Sarah falls down a hole and is knocked unconscious. She awakens, manages to escape from the cave, runs to her vehicle, and speeds off. Upon seeing Juno sitting next to her, Sarah screams and reawakens to find herself still in the cavern, revealing that the events since her previous awakening were a dream. She hallucinates about a smiling Jessica close by her, just as the camera pans out revealing that Sarah has reached a dead end, and the calls of the crawlers are growing louder.
=== Alternate versions ===
For the U.S. theatrical release, the film ends with the appearance of Juno in the car, and the scene where Sarah hallucinates her daughter in the torchlight was removed entirely. The Unrated DVD released in the US includes the original ending. This ending leads to the events of the next film.
violence, revenge, sadist