Juno (Natalie Mendoza), Sarah (Shauna Macdonald) and Beth (Alex Reid) are whitewater rafting in Scotland. Sarah's husband Paul (Oliver Milburn) and their daughter Jessica (Molly Kayll) wave and cheer from the bank. On the drive back to their hotel, Paul is distracted, causing a head-on collision. Paul and Jessica are killed instantly, but Sarah survives.
One year later, Juno, Sarah, Beth, Sam (MyAnna Buring) and Rebecca (Saskia Mulder) are reunited at a rustic cabin in the Appalachian Mountains of North Carolina, USA. Holly (Nora-Jane Noone), Juno's new friend, is introduced. As they reminisce over an old photo of Juno, Sarah, and Beth, Sarah says "Love each day", explaining that it was a saying of her late husband's. The next morning the group goes caving. When the group breaks for lunch in a huge gallery, Juno tearfully apologizes to Sarah for not being around for her after the accident, but Sarah is distant. As the group moves through the next passage it collapses behind them, with Sarah barely making it through. After a heated discussion, Juno admits she has led them into an unknown cave system, instead of the fully explored cave system they had originally planned for. The only people who were told about their expedition think they are at the other cave system, making rescue impossible. They are trapped with no way out. Privately, Juno tells Sarah that she led them into the unknown cave hoping to restore their relationship, but Sarah rebuffs her. The group discovers both a cave painting and climbing equipment from a previous visitor, suggesting a second exit exists. Juno keeps the latter secret, allowing for the group to remain hopeful.
Holly falls down a hole and breaks her leg. Sam sets Holly's fracture with a splint and they carry her along. As the others help Holly, Sarah wanders off and observes a pale, humanoid creature drinking at a pool. It scampers off into the darkness when Sarah gasps. The others think Sarah imagined it, but Sarah insists that she saw someone. Soon after, they are attacked by one of the creatures, referred to as "mole people". The group scatter, and the crawler rips Holly's throat. Sarah trips and falls and passes out. Seeing Holly is still alive, Juno tries to defend her from the crawlers, but in the confusion, she whirls around, only to stab Beth through the neck with her pickaxe. Beth grabs Juno's pendant as she drops to the ground, but Juno stumbles away in shock as Beth reaches out to her. Juno eventually locates Sam and Rebecca and rescues them from a crawler. Juno tells them she may have found a way out, but will not leave without Sarah. The others reluctantly agree to help her search. Meanwhile, Sarah awakens and encounters the mortally wounded Beth, who tells Sarah that Juno wounded her and left her. Sarah does not believe her until Beth gives her Juno's pendant, which is inscribed with the words "love each day". Beth reveals that Sarah's husband was having an affair with Juno. Beth, in extreme pain, asks Sarah to euthanize her, and Sarah reluctantly complies. Sarah soon encounters and kills a young crawler, a female crawler, and a male crawler in quick succession. Elsewhere, Juno, Sam and Rebecca are pursued by a large group of crawlers. Crawlers kill Sam and Rebecca, and Juno leaps into a chasm to escape.
Juno climbs out of the chasm and is helped onto a ledge by Sarah, who asks her if she saw Beth die. Juno nods. The two cautiously explore the caves until they encounter a group of crawlers and defeat them. Sarah then faces Juno, and reveals that she has Juno's pendant, revealing that she knows that she wounded Beth. Sarah cripples Juno with a pickaxe. Juno pulls the pickaxe from her leg and turns to face a large group of crawlers while Sarah leaves her behind. She hears Juno's screams, which abruptly fall silent. Sarah falls down a hole and is knocked unconscious. She awakens, scrambles up a huge pile of bones towards daylight, squeezes through a narrow opening onto the surface, runs to her vehicle and speeds off. She pulls over to vomit and sees Juno sitting next to her, her face streaked with blood. Sarah screams and awakens to find herself still in the cavern. She "sees" her long-dead, smiling daughter close by and a birthday cake between them. The field of view widens to reveal that Sarah is hallucinating and she is actually staring at a torch. The calls of the crawlers grow louder, but Sarah is oblivious.