The film directly follows the events of its predecessor, Hatchet II, starting with Marybeth Dunston (Danielle Harris) decisively eliminating Victor Crowley (Kane Hodder) by blowing his head off with a shotgun. As she walks away, she stumbles upon the lifeless bodies of Vernon and John. Hearing Crowley’s menacing presence in the woods, Marybeth revs up a chainsaw, only to be grabbed by Victor. After a fierce struggle, she kicks him, sending him crashing onto the chainsaw, which gruesomely splits him in half. Grabbing a shotgun from one of the victims, she finishes him off with another shot to the head before heading back to the city.
Upon her arrival at the Jefferson Parish Police Department, Marybeth is met with hostility as officers draw their weapons on her. The grim discovery of twenty to thirty bodies at Honey Island Swamp leads Sheriff Fowler (Zach Galligan) to declare her the prime suspect, despite her detailed accounts of the horrific events from the past two films. Sheriff Fowler heads to the swamp with paramedics and firefighters, leaving Deputy Winslow (Robert Diago DoQui) to manage the station.
Amanda Fowler (Caroline Williams), the Sheriff’s ex-wife and an expert on Victor Crowley, arrives at the station to conduct an interview with Marybeth. She insists on assisting her, warning that without her help, Marybeth could face execution for the murders linked to Crowley. As Marybeth recounts her harrowing experiences, Amanda reveals that Victor is cursed to relive the night of his death, perpetually searching for his deceased father. She informs Marybeth that her own father, Sampson Dunston, had initiated the fire that led to Victor’s tragic fate and suggests that only Marybeth can put an end to Victor's reign of terror.
At the swamp, Victor’s corpse is bagged and tagged for transport. However, as paramedic Randy (Sean Whalen) is distracted, Victor abruptly reanimates, violently dispatching him with a defibrillator. As Victor escapes back to his home, Deputy Hamilton (Jason Trost) investigates the scene, only to become another victim as Victor brutally kills him. Over the course of the night, ten additional fire department personnel and paramedics fall prey to Victor's wrath, leaving only Andrew (Parry Shen), Rick, and another paramedic alive to face their doom.
The arrival of the SWAT team, led by Hawes (Derek Mears), begins with high hopes, but soon descends into chaos as Crowley takes them down one by one. In a desperate attempt to fight back, Deputy Schneiderman (Cody Blue Snider) launches a rocket at Crowley, only to inadvertently kill Rick and ignite an explosion that collapses Crowley’s house around him. Yet, against all odds, Victor rises from the debris, exacting further revenge with merciless brutality.
Back at the station, with news of the SWAT team's grim fate echoing through the radio, Amanda persuades Deputy Winslow to release Marybeth so they can save those left at Honey Island Swamp. They seek the assistance of Abbott McMullen (Sid Haig, a distant relative of Victor who possesses the ashes of Thomas Crowley. When Abbott refuses to hand over the ashes, Amanda takes matters into her own hands, holding him at gunpoint to seize them.
Meanwhile, as chaos unfolds, another paramedic finds himself pursued by Victor, ultimately meeting a gruesome end. As Victor closes in on Sheriff Fowler, Andrew, and Dougherty, they barricade themselves in an ambulance, desperately calling for help from the National Guard. However, with Victor relentlessly sawing his way in, tension mounts.
Arriving at the Crowley house, Amanda attempts to lure Victor by calling for him with promises of his father's ashes. In a horrifying twist, Sheriff Fowler is snatched by Victor and dismembered. Trapped in the ambulance, Dougherty faces a cruel fate as she is dragged through the door by Victor.
The climax takes place at Crowley’s home as Marybeth confronts Victor, offering him his father's ashes in a bid for peace. As Victor reaches for the ashes, chaos erupts—Deputy Winslow is torn apart, and Amanda is beheaded while trying to help. In a final act of defiance, Marybeth smashes the urn over Victor’s head, scattering the ashes that cause him to disintegrate. With the last of her strength, she turns a weapon on Victor, ensuring he is finally defeated.
As the National Guard arrives on the scene and Andrew emerges from the boat, signaling to the helicopters overhead, the film ends on an ambiguous note with a fleeting glimpse of Marybeth gasping for air, leaving her ultimate fate uncertain and lingering in the audience's mind.
