The film picks up immediately after the end of the last film 'Hatchet II', with Marybeth Dunston (Danielle Harris) blowing off the head of Victor Crowley (Kane Hodder) with a shotgun. She walks away and comes across the bodies of Vernon and John, where she hears Crowley in the woods. Marybeth starts up the chainsaw, but is grabbed by Victor. Marybeth kicks him, and he falls backwards onto the chainsaw, cutting him down the middle. She grabs a shotgun from one of the men and shoots Victor's head, and walks back into the city.Marybeth walks into the Jefferson Parish Police Department, immediately having guns drawn on her. Upon discovery of the twenty to thirty bodies at Honey Island Swamp, she is placed as the prime suspect in the murders by Sheriff Fowler (Zach Galligan), even after telling him numerous times about the events of the last two films. The Sheriff heads out to the swamp with the paramedics and fire department, leaving Deputy Winslow (Robert Diago DoQui) in charge of the station until he gets back.Amanda Fowler (Caroline Williams), the Sheriff's ex-wife, journalist and Victor Crowley expert, comes into the station to interview Marybeth about what happened, telling Marybeth she wants to help, otherwise Marybeth will be tried and executed for what happened at Honey Island Swamp. After retelling the events of the previous two films to Amanda, Amanda tells her that Victor Crowley is a repeater, set to relive the night he died, looking for his father and that he will keep coming back unless he gets what he wants: his father. She tells Marybeth that her father, Sampson Dunston, was the one that came up with the idea to start the fire many years ago and tried to get him to deliver Thomas Crowley's ashes to Victor over the past several years, only to be pushed to the side numerous times. Since he's dead and she's Sampson's bloodline, only she can put an end to Victor Crowley. Marybeth doesn't believe her, continuously declaring that she killed Victor Crowley the night before.At Honey Island Swamp, the body of Victor Crowley is bagged and tagged and put into the ambulance boat. When the paramedic, Randy (Sean Whalen) is distracted, Victor reanimates and blows Randy's head off with a defibrillator. Victor then escapes back to his house. Deputy Hamilton (Jason Trost) goes to investigate and goes to the boat where Victor appears and kills him by hacking his head in half. Victor then kills 10 other fire department officials and paramedics, leaving Andrew (Parry Shen), Rick and another missing paramedic alive. In comes the SWAT team, led by Hawes (Derek Mears) and consists of six other armed men and one armed woman named Dougherty (Rileah Vanderbilt). Hawes takes over the operation from Fowler and leads them to the Crowley house, where Deputy Elbert (Diane Ayala Goldner) and another Deputy are picked off by Crowley. Crowley ambushes the SWAT team and kills them all. Hawes tackles Crowley but is in turn pinned against the shed and has his spine torn out. While Rick catches Victor's attention, Deputy Schneiderman (Cody Blue Snider) takes the opportunity to fire a rocket launcher at Crowley, but misses and kills Rick, and blows up the Crowley house, destroying it, covering Victor in the rubble. When Schneiderman begins to celebrate, Victor appears from the wreckage and throws a piece of rubble into Schneiderman's back. He then proceeds to kills Schneiderman by ripping his arms off and drowning him in a puddle. Dougherty, Andrew and the Sheriff run for it. Before running, the Sheriff manages to slow Crowley down by shooting in the chest with a handgun.Back at the station, upon hearing the SWAT team getting killed on the radio, Amanda convinces Deputy Winslow to let Marybeth out of jail and help her save everyone at Honey Island Swamp. She has Winslow take her and Marybeth to the house of Abbott McMullen (Sid Haig), Victor Crowley's racist long distant cousin and only living blood relative who possesses the ashes of Thomas Crowley. When he declines to give up the ashes, Amanda holds him at gunpoint and leaves with the ashes.Back at Honey Island Swamp, the other paramedic is chased by Victor Crowley when he hides behind the canoe from the first movie. There, he finds Ben (Joel David Moore), who is still alive, lying in the canoe, clinging to life with his left arm ripped off. As he begs for help, Victor throws the hatchet at the paramedic, misses and hits Ben in the face, finally killing him. As Victor advances towards the paramedic, an alligator comes out of the swamp and pulls the paramedic into the swamp.Victor resumes chasing the Sheriff, Andrew and Dougherty and corners them in the water ambulance. They barricade themselves in and the Sheriff calls for the National Guard on the ambulance's radio, telling them they're being attacked by crazed gunmen. The National Guard tells them they'll be there in ten minutes. Just as they settle down and think everything's all right, Victor begins to saw his way through the boat wall with the gas powered belt sander. The Sheriff tells them not to move, if they try to leave, they'll get killed, while Andrew thinks they should leave while he's busy cutting through the wall.Amanda, Deputy Winslow and Marybeth arrive at the Swamp outside the burned down Crowley house. Amanda calls out for Victor, telling him they have his father. Upon hearing his wife's voice, Sheriff Fowler attempts to leave, only to be grabbed by Victor and beheaded with the belt sander. Andrew and Dougherty are trapped in the boat with Victor on the other side of the broken door. Dougherty attempts to slowly get the Sheriff's gun, but Victor grabs and pulls her through the hole on the door, which disembowels her. Andrew remains in the boat, keeping himself out of sight of Victor.Victor goes back to his home, finding Marybeth, Amanda and Deputy Winslow there. Marybeth offers Victor his father's ashes and apologizes for what her father did to him. Victor proclaims "Daddy" upon seeing the ashes. When he tries to take the ashes, Deputy Winslow shoots Victor, but has his chest ripped apart by Victor. In response, Amanda attempts to take the urn from Marybeth, but is beheaded. Victor then knocks Marybeth over and impales her on a tree branch. Just when Victor was about to pick up a machete to finish Marybeth off, Marybeth smashes the urn over Victor's head, spreading his father's ashes all over him, and causing him to melt. With the last bit of her strength, she grabs one of the SWAT team's guns and blows Crowley away. The National Guard then arrives and Andrew emerges from the boat and signals the helicopters. Just as the screen cuts to black, one final shot of Marybeth gasping for air is shown, leaving her fate ambiguous.