Five years after the events of Freddy vs. Jason, Lori Campbell and Will Rollins return to Camp Crystal Lake to make sure Freddy and Jason are dead. They split up while looking around, and when Lori finds Will, he has been gutted by Jason. Jason eventually kills Lori by splitting her head open with his machete.
After his death in Freddy vs. Jason, Freddy is trapped inside Jason's mind, and is unable to use any of his power, except to appear in Jason's dreams, again creating a vision of his mother Mrs. Voorhees, telling him that Freddy is his new father, and if he wants to get him out of his mind anyway, he must go to the Voorhees house and get the Necronomicon Ex-Mortis, the Book of the Dead.
Ash is called to the S-Mart at Camp Crystal Lake. He meets three girls when he arrives. After Reginald, the manager, takes Ash inside to work, Jason kills the girls. Reginald introduces Ash to the housewares crew, consisting of Raoul, Jarvis, Dave, and Caroline. When the girls' bodies are discovered, Ash suspects that the Deadites killed them. Caroline tells Ash all about Jason.
After killing five more people, two boys (one named Furious, the other unnamed) and two girls (Bree and Becka) go over to the Voorhees house. Ash and Caroline go to the Voorhees house as well. Jason kills Becka, Furious, and Bree's boyfriend, while Ash finds the Necronomicon. Ash and Bree are attacked by Jason, who wrecks Ash's car, but they get into Caroline's car and she drives them away from the house.
Freddy discovers Ash's existence in Jason's mind, and they decide to go after him and get the Necronomicon. When they return to S-Mart, Ash, Caroline, and Bree tell Raoul, Jarvis, and Dave about Jason. Jason arrives at S-Mart and kills Reginald, along with many customers. Ash tries to fight him off, but Jason throws him through a wall, kills Bree, and takes the Necronomicon. Freddy reads the words "Klattu Verata Nikto" in the Necronomicon and resurrects himself. As part of his deal with Jason, he uses it to make Jason smarter.
Ash, Caroline, Raoul, Jarvis, and Dave fall asleep. Ash is attacked by Freddy, but is able to wake up. He realizes Freddy's power from a burn he got on his arm, and wakes everyone up except Dave, whom they are unable to wake up. Freddy kills Dave by shooting a ball through his chest. Freddy decides that Ash will come to the Voorhees house to kill them, and tells Jason to kill them first.
Ash, Caroline, Raoul, and Jarvis get some supplies and set a trap for Jason at the Voorhees house, but he does not fall for it. Ash finds the Necronomicon and is about to find a way to banish Jason to the Deadite dimension when Freddy appears, now able to use his powers in the real world because of the Necronomicon. Freddy attacks Ash, when suddenly Jason picks a fight with him, so he uses the Necronomicon to resurrect Jason's previous victims as Deadites, and they all fight Jason.
During the battle, Ash takes the Necronomicon and, after Freddy tries to stop him, discovers the bodies of Raoul and Jarvis, killed by Jason. He plans to use the Necronomicon to banish Freddy and Jason to the Deadite dimension. Ash and Caroline read it and discover that Mrs. Voorhees used the Necronomicon to resurrect Jason, giving him his power, indicating that Jason is an ultra-powerful kind of Deadite. Ash distracts Freddy so Caroline can banish the two slashers, and fights Freddy and Jason, shooting Jason's mask off. Freddy throws Ash through the window, supposedly killing him.
Freddy then goes after Caroline, but Jason appears before he can kill her. Freddy and Jason fight, and Ash returns, hitting Freddy with his car. Ash, Jason, and Freddy engage in a bloody 3-way battle until Caroline uses the Necronomicon to open a portal to the Deadite dimension. Freddy and the Necronomicon are sucked into it, but Jason is too strong, however, the car is affected by the suction, knocking Jason into the frozen Crystal Lake, freezing him under the ice.
Afterward, the portal spits out the Necronomicon, which opens to reveal a page showing Freddy, Jason, and Ash confronting each other.
|#||Name||Cause of death||Killer||On-screen||Notes|
|1.||Will Rollins||Gutted||Jason Voorhees||No|
|2.||Lori Campbell||Head split open||Jason Voorhees||Yes|
|3.||Paris||Chopped in two||Jason Voorhees||Yes|
|4.||Girl||Head banged together with other girl||Jason Voorhees||Yes||Double kill|
|5.||Girl||Head banged together with other girl||Jason Voorhees||Yes||Double kill|
|6.||Jenny||Speared with sign post||Jason Voorhees||Yes|
|7.||Conductor||Speared with sign post||Jason Voorhees||Yes|
|8.||Shelly||Decapitated||Jason Voorhees||Yes||Triple beheading|
|9.||Keith||Decapitated||Jason Voorhees||Yes||Triple beheading|
|10.||Man||Decapitated||Jason Voorhees||Yes||Triple beheading|
|11.||Becka||Torn in half||Jason Voorhees||Yes|
|12.||Furious||Head crushed||Jason Voorhees||Yes|
|13.||Bree's boyfriend||Stabbed||Jason Voorhees||Yes|
|14.||Portly lady||Stabbed in mouth with fist||Jason Voorhees||Yes|
|15.||Man||Chopped in half||Jason Voorhees||Yes|
|16.||Old man||Shopping cart smashed on head||Jason Voorhees||Yes|
|17.||Woman||Impaled on sharp shelves||Jason Voorhees||Yes|
|18.||Unknown customer||Offscreen||Jason Voorhees||No|
|19.||Reginald||Head chopped in two at the mouth||Jason Voorhees||Yes|
|20.||11 people||Two decapitated, one chopped in two, eight stabbed or slashed||Jason Voorhees||No|
|21.||Bree||Throat torn out||Jason Voorhees||No|
|22.||Dave||Volleyballs shot through chest||Freddy Krueger||Yes|
|23.||Raoul||Chopped in two||Jason Voorhees||No|
|25.||Jason's previous victims, as Deadites||Offscreen||Jason Voorhees||No||Victims were Deadites resurrected by Freddy to fight Jason|