Eleven years ago, a cave-in on the north side of a mine trapped six miners. Six days later when rescue teams arrived, they found five dead miners and Harry Warden, who survived by killing the other miners with a pick-axe, in a coma. Tom Hanniger is blamed for the mine disaster because he forgot to vent the methane lines.
The following year on Valentine's Day, Warden wakes up from his coma, killing many in the process. At the abandoned mineshaft that was the site of the disaster, a party is in full swing, attended by many teens, including Axel, his girlfriend Irene, Tom Hanniger, and his girlfriend, Sarah. Sarah goes in alone and gets lost looking for Axel and Irene. She runs across a teen and a few seconds later he is stabbed in the back of his head through his eye. She is confronted by Harry Warden in full miner's garb, carrying a bloody pick-axe, and flees for safety; Axel grabs her and they along with Irene hide from the killer. He eventually sees Sarah, Axel and Irene, and they run out of the mine, where they run into Tom coming in. The killer hits him with the pick axe, injuring him, while the other three run for the car and leave him behind. Tom runs back into the mine in an attempt to get away from the killer; just as he's about to be killed, the police arrive and shoot the killer, but he makes his getaway back into the mine.
Ten years later, Tom's father, from who Tom was estranged, dies and Tom inherits the mine. Tom then returns to town after his father's funeral to sell the mine. Axel is now sheriff and married to Sarah, however he is cheating on her with Megan, who helps out his wife at her grocery store, who admits to Axel after they finish having sex, that she is pregnant with his child. In quick succession, Irene, who is now a prostitute along with a trucker, is murdered by Harry Warden at the motel where Tom is staying, then as Tom goes to check out his newly inherited mine, a miner is murdered. While in the mine, when Warden appears, Tom is forced into a cage and Warden bends the metal latch on the door; making Tom's escape impossible. Warden then brutally murders the miner who was accompanying Tom on his journey into the mine, as Tom is forced to watch. As the rest of the mining crew arrive to see what is the matter, Warden flees, and suspicion is cast on Tom; despite the fact that he was locked in a cage the entire time. Sheriff Axel then asks his father, the retired sheriff, what happened to Warden. He says that he and Tom's father killed and buried him. Axel, Tom, Sarah and Axel's father go to the spot where Warden was buried to see that his body is no longer there.
Next, the current mine manager and Megan are killed by Warden. Tom picks Sarah up to take her to Axel's love nest to convince her that Axel is the killer. Axel then calls her and tells her that Tom is the killer. She crashes Tom's car and makes her way to Axel's shack. Sarah is chased by the killer all the way to the mine. There, Sarah goes into the shaft where the original murders took place, where she is joined by both Tom and Axel, and after a brief stand off, Axel and Sarah realize that Tom is delusional. A montage then plays out all the murders in the film again; this time showing the aftermath and beforehand, revealing Tom to be the killer. The killing in the mine in which Tom was locked in the cage, is played out again, this time showing Tom commit the murder, then lock himself in the cage; revealing Warden locking him in to be a hallucination. Sarah shoots a tank and causes an explosion, and subsequently a partial cave-in. She and Axel escape, believing and reporting Tom to be dead. A rescue team comes in to the mine to look for survivors, but Tom kills one of the rescuers and escapes the scene dressed in the rescue worker's clothing.