The movie starts with radio broadcasts and police communications going back and forth. Through them we learn that there has been a cave-in at the north side of Hanniger Mining Co. Eventually, rescue teams find six people in the shaft -- five of them dead and one of them, Harry Warden (Richard John Walters), in a coma. The local papers are soon accusing Harry of having gone crazy and murdering the other five miners to prolong his air supply, and given the condition of some bodies there is enough evidence to support this. A few months after the accident, on Valentine's Day, we see Harry's brain remembering the last few minutes before the collapse, which consist of him telling Tom (Jensen Ackles), the son of the boss, to not screw up and threatening him a little. Something goes wrong, however, and soon everything is collapsing and Tom barely makes it out alive. Suddenly Harry wakes up and looks around, trying to bring his memory up to date. A nurse enters the room and before she can sound the alarm, gets killed by Harry.
Not long after, we see Sheriff Burke (Tom Atkins) and one of his deputies touring the hospital massacre scene (makes the Saw movies look like a Lassie film). They enter Harry's room and see him missing and a nurse with her heart removed and on the counter beneath a message reading "HAPPY VALENTINE'S DAY" and are soon giving chase, figuring that he'd head to the mine. Meanwhile, over at said mine, a big senior party is in full-swing for the high-school set. One of the kids, Axel (Kerr Smith), is making out with his girlfriend Irene (Betsy Rue) while waiting for her friend Sarah to arrive, although Axel isn't excited since she'll be bringing Tom in all likelihood. Sure enough, Sarah (Jaime King) does arrive with Tom and the boys are naturally cool towards each other. Axel and Irene head into the mine with Axel telling her that he doesn't trust Tom as Sarah and Tom hang back a second. Tom reassures Sarah that he's fine with being there and they go in. The foursome splits up (as one does in "haunted" houses) and Sarah is soon lost. She calls out for Tom and is surprised by some kid named Jason. He calls for his buddy but instead gets pickaxed from behind by Harry. Sarah screams and runs, but Harry has already been through, killing nearly everyone. She runs into Axel and Irene and the three of them hide just as Jason's friend comes up. The friend inadvertently reveals them before getting killed and the trio heads for the front entrance. They pass Tom and drag him along, but Tom trips on some wires and gets abandoned. Axel shoves the girls into his car despite Sarah's attempts to get to Tom just as Harry throws a pickaxe at them, which comes within centimeters of impaling Sarah (it gets stuck in the windshield). They back out of there just as Burke and his deputy arrive. The two of them shoot at Harry just before he can kill Tom, and Harry heads deeper into the mine as Burke and the deputy give chase...
Ten years later, we enter in on now-sheriff Axel having lunch in a diner while watching an interview of himself on TV regarding the ten-year anniversary of the "Valentine's Day Massacre", during which he loudly and profanely tells the reporters to, essentially, get a life (note that that's watered way down). One of the customers, miner Ben Foley (Kevin Tighe), starts sarcastically congratulating Axel about the interview just as one of Axel's deputies, Martin (Edi Gathegi) comes in and pulls him out to work. As he's driving, Axel gets a call from his wife, Sarah (yes, Tom's Sarah), and they talk for a few minutes before she gets back to her job working at a local grocery store (not chain, just....local). A little later, we see Axel pulled up outside his dad's cabin and we see that inside he is having sex with Sarah's co-worker Megan (Megan Boone). She tries to give him a box of Valentine's chocolates with a note that says "Be Mine 4 Ever", and he accepts as graciously as a married man wanting to keep a secret could.
Not far away, we see Tom looking out over the mine, watching the workers do their thing. That night he goes over to Ben's house and is told that they're not selling until Monday, which Tom is less than pleased with. Ben says that Tom's dad would be disappointed that Tom is selling the mine, but Tom doesn't care -- he just wants to sell the place and get back out of town. He goes to a motel and grabs a room from the dwarf hotel manager, inadvertently passing a room where he sees Irene and a trucker having very loud sex....in the room right next to his. As Tom tries to shut them out, Irene and the trucker finish up. Irene (completely naked) starts waxing philosophic about Valentine's Day and the Romans (short version: the males were randomly assigned secret sex slaves for the next year by pulling the girls names out of a hat....or something like that). Frank doesn't care and pulls a camera out of his bag, showing that he taped it, and throws some money at Irene, calling her a whore and walking out. Irene throws the money at the door, goes into her bathroom, and grabs a gun from under the sink. She walks into the parking lot (still naked) and demands the tape from Frank, who calls her bluff by pointing out the lack of bullets, which in turn makes her use the gun as a projectile weapon (she throws it at his head and scores a bulls-eye). He curses and opens the truck door...and gets pickaxed by a guy in a miner outfit. Irene screams and runs for the phone in the office, just barely getting through to the cops before moving to hide in the manager's room. She gets under the bed (still naked) just as the miner enters the room. He looks around but doesn't see her in the closet and is about to leave before the manager starts calling for her pug dog. She looks in the closet and gets pickaxed through half her body and into the overhead lights. Irene lets out a squeal and is discovered. She manages to keep the bed frame between her and the miner while carrying it towards the door with her (not bolted down), but eventually she loses it and is pickaxed through the stomach....again....and again....
Not long after, Axel and all his cops are there, including deputies Martin and Ferris (Karen Baum). They look at the mutilated bodies and discover something missing -- Irene's heart. They check the guest log and find Tom's name as the last person who registered for a room. The investigation continues into the next day, when Axel gets a valentine from a secret admirer....and inside it is Irene's heart. Meanwhile, at Sarah's store, she is working the register when Tom comes up to her, wanting to get together with her and talk things out, although she is less than willing to, what with being married. That night, as Axel investigates and Sarah works, we see Tom getting into a bar fight with all the miners who will now be out of work once the mine closes...although the real cause is more the fact that they think he had something to do with the murders. Foley and Burke step in, and Burke tells Tom to get out of there, since he is unwilling to save Tom's life a third time. Later that night, Sarah is putting her and Tom's son Noah to bed and sending their house sitter Nora home when she hears Axel watching the tape from the hotel. He tells her what happened and mentions how Tom is a suspect and some people are saying that Harry Warden is back from the dead. When she tells him that Tom spoke to her earlier at the store, he cautions her and tells her to call him if Tom tries to get in touch with her again.
The next morning, Sarah is opening the store when Tom pops up behind her. They take a brief walk and Sarah explains that she can't be with Tom because he spent the last ten years running. She leaves him and he heads up to the mine, determined to talk to Ben and tell him that he's not selling and is staying. He is told that Ben is currently in tunnel 3 and, after being outfitted, is driven down there by a guy named Red. They stop at the main intersection and Red rings 3 as Tom heads to some electrical equipment behind a chain-link door. Something shoves him in there and he turns and sees the miner, which terrifies him into silence. The miner gestures with his pickaxe for him to be quiet and then wedges the lock shut. The miner turns and heads for Red, who manages to see him and fights him off as over the phone the other miners hear his shouts and start converging. The miner gets the upper hand and drives the axe through Red's forehead before starting an attempt to take out his heart. This gets interrupted by the approaching miners, though, and the miner runs off just as the rest of the crew comes running up, at first thinking that it's Tom's fault before saner heads point out how he was stuck in there. They snap the lock open and Tom is taken to a hospital where he gets stitched up while telling his story to Axel, who doesn't believe a word of it, since he knows Harry Warden is dead. Ben, who was stuck in tunnel 3, finally arrives and Axel reveals that he knows what Harry and the sheriff and the other town fathers did with Harry -- namely, dispatch him vigilante-style and bury him in the woods. Less than two hours later, Axel, Sarah, Martin, Tom, Ben, and Burke are hiking through the woods towards Harry's gravesite. Burke is explaining that a bunch of them got together after the massacre and tracked him down into the mine. They shot him full of bullets and then gave him a better burial then he deserved (according to Burke), so he is definitely dead. They come upon an exit from one of the mine tunnels, which Ben and Burke identify as the site but the site has already been unburied.
Back at the police station, Axel is interrogating Tom, but Tom is sticking to his story and saying that he's probably gonna stay in town for good this time. Axel gets upset and starts throwing stuff around but Martin breaks it up. Tom is let go and both he and Axel make vows (Tom to Sarah, Axel to Martin) that they will find the guy doing this.
That night, we see Tom hanging around the mines after dark. He approaches the exterior entrance to Tunnel 5. He looks inside and sees....nothing. He steps back outside but the door doesn't close all the way and it creaks open again. He stares back at it but before he can do anything further he sees the miner watching him from the forest. The pickaxe gets pointed at him but the miner simply turns and walks away. Tom gives chase and eventually is led to Axel's cabin, of all places. Tom goes in the open door and looks around. Meanwhile, Ben is drunk at home and playing with a shotgun (it's unloaded) when someone knocks on his door. He yells for who it is but no one answers. He gets up and loads the shotgun. Cautiously, he looks around outside but sees nothing and laughs it off, cursing Harry for making him jump at shadows. He goes back inside and unloads the gun just as Harry/the miner steps out from the shadows. Ben turns and manages to get the shotgun up to defended against the axe. The shotgun gets batted away but Harry misses again as Ben rolls and the axe goes into the floor, pointy end up. Harry manages to get his hands around Ben and, after some more struggling, drives him into the axe. Over at the station, we see Axel getting more and more frustrated, especially since both Tom and Ben are missing-in-action. That record ends, though, when they get a call saying that Ben is dead. Axel orders Ferris over to his house to watch his kid and him and Martin head over to Harry's to check things out there.
Meanwhile, Megan and Sarah are busy closing up the grocery store when they hear a noise up front. After verifying that Megan did indeed lock the front door, they check it out and see that the door is still locked and that no one is there. Another noise sounds from in the back and they star looking up the aisles, just barely missing seeing Harry in the back. They split up and Sarah goes up the freezer aisle and is attacked by Harry. He puts his axe in one of the freezers and starts trying to get her but is distracted by Megan attacking him with a mop. The two girls run to the loading area and into the office, pursued by Harry and his axe. They shove a few things against the door as he tries to get through and Megan starts trying to unlock the exterior screen so they can escape through the window while Sarah quickly calls the cops. She gets it open and is halfway out before they realize Harry has gone. Sarah tries to pull Megan back in but Harry gets to her first and pulls her out. Sarah gets back out of the office and heads for the front door where Axel is just outside (suspicious?). He pulls her out and they head around the corner to find Megan's body flayed with the heart missing and the words "BE MINE 4 EVER" scrawled in blood above her body. A little later, Sarah is being treated by an ambulance team and is about to be taken to a hospital when she tells Axel that she knows that he and Megan were sleeping together.
Elsewhere, we see things unfolding pretty much normally at the Palmer household, with Noah watching cartoons and Nora doing laundry....until Harry sneaks up behind Nora and makes quick work of her. Outside, Ferris is in her car almost asleep when Burke knocks on the window and tells her that Harry is inside. Ferris takes point and finds Noah crouched behind the couch and Nora in the washing machine while Burke tries to secure the porch and instead gets axed. Over at the hospital, Sarah is getting her wounds bandaged when she gets a call. She answers and hears Tom on the other end, telling her not to trust Axel and that he found something he needs to show her. He picks her up from the hospital just as Axel gets a call from Martin regarding something about Tom. As Tom is driving Sarah up to the mine and stressing how sure he is that Axel is doing all the killing, Sarah gets a call from Axel. Tom tells her not to answer but she does, and Axel tells her that Martin found out Tom was in a mental institution for seven years. He tells her to try to get away and then call him and they quickly hang up. She tries to tell Tom to stop and when that doesn't work she grabs they wheel and yanks it so hard that the car impales itself on an overturned tree branch. Tom gets knocked out momentarily, and Sarah escapes. She calls Axel and he tells her to go to his dad's old cabin (the one Tom was at earlier). She gets there just as Tom finally crawls out of the car and inside she finds about three hundred heart-shaped candy boxes and one miner trying to kill her. She runs for the front door but can't get it open so she goes with the option of diving out a second-story window onto the sloping roof and running from there to the mine.
At the mine, Laura runs into the wide-open Tunnel 5 entrance and heads into a locker room where she quickly gets lost among the maze of falling miner outfits. The miner shows up and starts hacking through them and she goes out the other end and into the mine proper. She runs throughout the tunnels and runs into Axel. She steals his gun and tells him about the boxes, but he thinks (at first) that she's just talking about the one from Megan and is confused when she points out the other three hundred. Tom comes out a side tunnel and she points the gun at him too, although both are saying the other is the killer. Axel suggests shooting them both and Sarah seems about to do it when Tom points out that "Be Mine 4 Ever", the message over Megan's body, was the same as what she wrote to Axel. Sarah stops breathing and slowly asks Tom how he knows about that, since she didn't tell him. Before he can answer he sees Harry behind Sarah and tries to warn her, but she doesn't believe him and Harry....walks right through her. She tells him that Harry isn't there and Axel realizes that he is there....inside Tom, and we now see Tom digging up Harry's body and taking off the outfit after doing the killings and attacking Irene and seeing Harry in the mirror (make your inevitable Fight Club comparisons now). Axel asks "Harry" if that's right, and Tom grins and says, "Oh yeah," before attacking Axel full-on. The two struggle and Axel gets a pickaxe through the gut (non-lethal). Sarah knocks Tom into some old crates and she and Axel run, hiding behind a corner like they did ten years ago. Tom gets it together and starts for them, knocking out lights as he goes. Axel gives Sarah his gun with one bullet and tells her not to miss. She waits until Tom is right by some gas tanks and then fires. The bullet hits home and then goes right through Tom, hitting the tanks and making them explode and the walls collapse.
Some time later, we see a bunch of miners searching the rubble for any survivors. One guy thinks he sees something and heads on ahead. He starts talking to the victim he found, Tom, and telling him he'll be okay. Tom stares at the guy, seemingly not comprehending, and then he picks up a pickaxe and drives it through the guy’s skull. Outside, Sarah and Axel are being brought out -- her on her feet, him on a stretcher. They tell each other they love each other as Martin approaches. He asks them what happened to Tom and they tell him that Tom is dead. As the ambulance drives away, the rescue team comes out, all accounted for. They start to disperse, heading their separate ways, and one of them pauses at the top of the hill for a look around. He takes off his mask, and glances around. Tom nods, satisfied, and then heads off into the forest...
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