Instead of being sucked into the vortex at the end of Evil Dead II, Ash has been placed in a mental institution for the criminally insane as a result of the events of The Evil Dead and Evil Dead II. His doctor has gone mad and obtained the Necronomicon Ex Mortis and releases the deadites once again.
The game begins in Sunny Meadows, an asylum Ash is committed to. Ash is briefly visited by his lawyer, Sally. Sally talks about how she found Professor Knowby's diary and believes Ash's story about the Necronomicon and the cabin. Ash thanks her, but just as she is about to leave Ash comments on her wardrobe and advises her to wear more revealing clothing. Just as Sally leaves, Reinhard accidentally unleashes the dead through the Necronomicon. The evil also breaks Ash free in the process. Ash travels through the asylum killing deadites with a pistol he retrieved from a dead security guard. He eventually finds his clothing, his boomstick and his chainsaw in the process. As he goes through the asylum, Ash finds an electric deadite named Sparky. Ash kills Sparky and decapitates him. Ash briefly breaks the fourth wall as he speaks to the camera and throws Sparky's head away. Ash proceeds through the nearby cellar. However, just as he is about to leave the asylum, Professor Knowby appears to Ash and tells him about Reinhard and what he plans to do. Ash, however, refuses to try and stop him and begins to leave. Professor Knowby also speaks about how Sally is in danger for taking his diary. Ash decides to stop Reinhard just to save Sally. Professor Knowby talks about how he needs to close deadite portals, but he cannot do it alone. Knowby introduces Ash to Sam, a short half deadite who will respawn if killed, and Ash quickly makes a bad first impression on him by shooting him dead. Sam knows all the necessary incantations to close the portals. Ash and Sam travel through to a cemetery and eventually find a portal in the nearby catacombs. However guarding the portal is a large deadite queen. Ash kills the queen with Sam's help and Sam closes the portal.
Ash and Sam go through the woods surrounding Sunny Meadows to find it infested with deadites as well. Ash finds a harpoon gun in a trash pile and modifies it onto his right hand. Ash and Sam, with the help of his harpoon gun, make their way into an abandoned mine where they find another portal. However, as soon as they get there a spirit possesses a miner. Ash and Sam kill the deadite miner and close the portal. Professor Knowby tells them that Reinhard has his diary finally and needs a human sacrifice. Ash believes that Sally is the one he intends to sacrifice. Ash and Sam follow a mine shaft towards nearby docks where another portal is believed to be located. Ash goes to another trash pile there and finds a high powered torpedo gun. Ash and Sam make their way to the portal to find it guarded by a giant fish creature who Ash calls "The one who got away." Ash kills the fish creature and Sam closes the portal. They both go through the nearby swamp.
Ash and Sam make their way through the swamp. Ash finds a shack full of flammable materials and creates a flamethrower. Ash and Sam make their way through the swamp to a nearby town called Port Turnham which is infested with Deadites. They make their way through the destroyed city and in the process Ash replaces his chainsaw with a newer one with a titanium blade. Ash and Sam find a portal in a nearby courthouse. However, Sam succumbs to fatigue and yawns while he is saying the incantation. The portal, instead of closing, sucks both of them into it. Once they are inside the portal Ash and Sam find themselves in a hellish dimension. Professor Knowby shows up telling them the only way out is to defeat Reinhard and rescue Sally. However, he begins talking about how Ash must save the world in which Ash becomes very angry and says he is only out to save Sally and maybe Sam.
Ash and Sam face many obstacles in the deadite temple. After much struggle they make their way to Reinhard's lair where he is just about to sacrifice Sally. Rienhard says how impressed he is with Ash. However, he apologizes that they had to come a long way to die. Rienhard uses the Necronomicon to turn himself into a troll-like deadite. Ash and Sam kill Rienhard and Sam closes the portal. In a very romantic scene Ash says he came a long way and overcame great odds to apologize to Sally on the fashion advice he gave her back at Sunny Meadows. Just as he is leaning to kiss Sally a spirit possesses her. Ash, overcome with rage, kills the possessed Sally. However, just as he does this the portal reopens and sucks Ash and Sam into it along with the Necronomicon which ends the game. This is possibly Ash being sent into Army of Darkness as a way to connect the game and the movies.
The gameplay mostly consists of hack and slash with small conveniently placed puzzles. The player takes the role of Ash Williams as he fights off hordes of Deadites. The player can find different kinds of weapons in levels which find use in different situations. Early on in the game Ash receives his sidekick Sam who fights along with him in battle. Unlike other games where killing a friend ingame is taboo at best, Sam actually comically dies many times in the game. The player can kick Sam into openings to open doors or onto enemies to pop their heads off. The player can also possess Sam to get through small openings; the purpose is to open a door or a switch. At times, Sam is controlled by the computer and destroys adversaries without assistance. Sometimes enemies will need a 'finishing move' (pressing a specific button) in order to completely destroy them.
In contrast with previous games, the game automatically gives the player unlimited ammunition for his weapons and unlimited gas to his chainsaw. A new gameplay element is rage mode. When the player kills enough enemies it fills up his rage meter. When the rage meter is filled up to a decent amount it allows Ash to go into "rage mode". Rage mode transforms Ash into a powerful deadite killing machine. It instantly doubles all of Ash's weapon damage and makes him lose health slower.
Through the game, Ash collects six weapons that can be used to kill the Deadites. Some are also used to pass environment puzzles, like wooden barricades or bridges.
Ash collects 3 guns throught the game and are collected in this order:
Pistol: Aquirred in Sunny Meadows Aslyum off of a dead gaurd who was killed by a possessed inmate (Ash says "Let's see if these freaks are still alergic to lead!" after he picks it up.). According to the game manual, it's loaded with .45 rounds and looks like an M1911 semi-auto pistol. Dependy on how fast is your trigger finger, this gun has a high rate of fire and a really short reload. Unfortunally, the pistol does not carry a large amount of rounds, so Ash will reload it alot. It is not as powerful as the Boomstick or the Firlance, but still has it's uses. For example, if you use the Harpoon Hand and the Pistol together, you can keep a hold on a deadite while blasting its face off.
Boomstick: Acquired also in Sunny Meadows Asylum in the inmates' belongings locker next to the morgue, along with Ash' chainsaw hand. The boomstick fires considerably slower than the pistol and does not seem to have a greater effect on enemies per shot. Ash seems to need to reload this a lot. In combos, Ash can perform interesting finishers, such as shooting a deadite behind him. However, you do not need to be using the boomstick when you trigger the combos; it will automatically change to and use the boomstick.
"Firelance": Aquired at the Port Turnham Shipyard in a trash pile, the second to last weapon that Ash collects in the game. The Firelance (while never refered as this in the game) fires miniture rockets that attach into the deadites. It literally blows away a large chunk of their health, but doesn't allways have a one shot kill. This weapon is required to kill "The one that got away". A useful combo is the firelance and the flame thrower.
Since Ash lost his right hand to the deadites, he decided to replace it with weapons. Ash collects 3 of them through the game, although technically he has 4.
Stump: Once you start the Sunny Meadows Aslyum level, Ash has no weapon except for the pistol and the stump where his right hand was. With the stump, Ash can only punch a deadite to make some room for Ash. It does no damage what so ever, it only stuns a deadite.
Chainsaw Hand: Aquired in the Sunny Meadows Aslyum in the inmates' belongings locker next to the morgue, along with Ash's boomstick. A useful weapon that can easily cut through almost any deadite foe. Ash collects two variants, the orginal red one and a new yellow chainsaw with a press and released, customized tune pipe and 50 inch titanium blade (Just like the one in Ash's dreams). Ash uses the red one up until the Port Turnham hardware store. After that, Ash has the yellow one and can't get the red one back (although, I don't see why you would want the old one back).
Harpoon Hand: Acquired on the outside of the mine. After finding some parts in a scrap pile, Ash assembled this weapon to act as a variant to his chainsaw hand. This handy weapon can be fired into Deadites to pull them in close so that Ash can pump some lead into them with his guns, very useful for getting at far away deadites. It is also used to pull on certain objects to move them, such as doors or mining carts. Flamethrower Hand: Acquired at the swamp area, cobbled from some parts Ash found in a shed to act as another variant for his missing hand. This can be used to send out a wave of fire that lights up Deadites. It is also used to light things on fire so as to get rid of obstacles, such as when Ash lit Sam on fire, the punted him into a building full of flammable gas to open the way to Port Turnham.
During his escape from the Asylum, Ash manages to recover his clothes and weapons (boomstick and chainsaw) from the Morgue. However, when the corpses change into deadites and awaken, Ash says "Yeah baby. Sing for the King" - despite the fact the game is set before Army of Darkness when he was offered to be King.
Evil Dead: Regeneration was met with mixed reviews. Aggregating review website GameRankings and Metacritic gave the PlayStation 2 version 69.17% and 68/100, the PC version 67.75% and 69/100 and the Xbox version 67.13% and 66/100.