The film opens with a truck driver coming across Sarah wandering in the woods, covered in blood. He takes her to the hospital and the local law enforcement are informed. They are busy overseeing a cave rescue operation focused on the harmless local caves where the women claimed they were going in the first movie; the sheriff is amazed that Sarah was found on the other side of the county. He sends a deputy with the trucker and a dog to track down where she came from whilst he and a deputy interview her. Sarah, suffering from amnesia, is unable to help.
The tracker dog leads the police to an old abandoned mine, and the trucker recalls his grandfather talking about breaking into a cave system from the mine many years ago, then vanishing when he was sent in to investigate. A small rescue party proceeds into the mine, with Sarah along to act as guide.
The party is quickly cut off by a cave-in and the monsters attack once more. Juno is found still alive in the caves, and although she and Sarah attack one another on meeting they eventually reconcile their differences. Juno leads Sarah and a surviving deputy to the cave exit, where the creatures feed. They are attacked there and Juno is killed. With the exit blocked, Sarah sacrifices herself to distract the creatures so the lone surviving deputy can escape.
Once out, the deputy attempts to call for help but is killed by the trucker with a spade. He drops her body back into the caves where a creature grabs at it. No clear explanation is provided for why the man does this.