Three friends - Sarah, Beth, and Juno - partake in a white water rapid expedition. The adventure ends safely and happily when the three women reach calmer water and Sarah's husband, Paul, and daughter, Jessica, who wait on the bank of the river. When no one is watching, Paul affectionately helps Juno with her helmet. Sarah leaves with her family, saying she will meet the others at the hotel. On the drive to the hotel, Sarah asks Paul if he is okay; Paul is temporarily distracted and collides head-on with a vehicle transporting copper poles. Both Paul and Jessica are impaled and killed by the poles, which Sarah discovers after waking up alone in a hospital bed. Sarah is haunted by dreams of her daughter blowing out candles on her birthday cake.
Sarah reunites with Beth and Juno for a caving expedition one year later in the Appalachian Mountains of North Carolina. They are joined by Holly, Juno's Scottish "protege" and sports enthusiast, as well as half-sisters Rebecca, an experienced climber, and Sam, a medical student who studies in America. After spending the night in a cabin, drinking and reminiscing, the women leave to explore a nearby cave (level 2 difficulty). The women head deeper into the cave, crawling through a narrow tunnel in which Sarah is almost trapped. The tunnel caves in behind Sarah, forcing her to abandon one of the group's bags of rope. Frightened and desperate, the group turns to Juno for direction. Juno reveals that she has left the guide book in the car, and Rebecca rejects the reassurance that local authorities will locate them in the event they don't return - Rebecca registered their intended expedition plans with Mountain Rescue, but Juno has deliberately led them to a different cave.
When crossing a deep chasm, the women find climbers' equipment left by past explorers. They quickly lose hope of escape, realizing that no explorer has successfully exited the cave because the cave remains unnamed (naming rights typically go to the first successful explorers). The women ultimately discover what has killed all explorers of the cave: an adapted form of cannibalistic human that locates prey (both human and animal) by sound.