After a quick recap of the events of Feast 2: Sloppy Seconds, the film picks up where the previous film left off. Honey Pie is mauled and beheaded by a monster who then devours her head and passes it within a few seconds. Lightning having survived the stick of dynamite from the end of Feast 2 gets up and stumbles off. The remaining survivors (Biker Queen, Tat Girl, Tit Girl, Secrets, Bartender, Slasher and Greg now with a pipe lodged in his head) are then attacked by monsters on the roof of the building they are hiding on, Biker Queen quickly dispatches of the monster, killing it with her motorcycle. Thunder is seen still alive and crawling away after being disemboweled and ripped in half by a pair of monsters, when another survivor only known as Shitkicker driving a Modified Military Jeep runs Thunder along with the two monsters over, killing them.
Shitkicker smashes open the front door of the police station and enters. The rooftop survivors make their way to the jail and gain entry, Hobo attempts but fails miserably to kill the survivors with a shotgun, Tat Girl throws her hammer at Hobo stunning him, and then Biker Queen along with Tat Girl immediately pounce on Hobo savagely attacking him (while Biker Queen questions if Hobo remembers her, "Remember me, Motherfucker!?") which causes Hobo's bowels to loosen in his pants with Slasher remarking "Damn, you two literally kicked the shit outta him." Greg, struggling to speak through the pipe, reveals to the rest of the survivors that Slasher has a lot of used cars that the survivors can use to escape. Shitkicker comes up with a plan to escape using Slasher's cars much to Slasher's dismay. Shitkicker is accidentally shot in the head by Secrets when he was showing her how to shoot a revolver. The gunshot alerts and causes the monsters to enter the police station. The remaining survivors decide to lock themselves inside the jail and send Slasher to retrieve the weapons. The monsters enter after Slasher tries to sneak into the front room to unlock the gun safe. After Slasher opens the safe the other survivors hear something and quickly start pulling the rope that is tied around Slasher. Slasher loses the keys inside the safe and is pulled back to the jail cells with the monsters right behind him. The survivors lock themselves inside the cell in an effort to protect themselves. They then decide to make a break for it and run to the used car lot.
Once they exit, Slasher runs a different way from the group, and then trips Hobo as a distraction for the monsters. While a monster begins to eat Hobo, Slasher runs towards a metal storage unit. The monster takes one bite of Hobo's leg, but spits it out in disgust. The monster then resumes to go after Slasher, who is joined by two of the biker women in the unit. Inside the unit, they meet five unknown survivors who proceed to gang up and beat Slasher for ripping them off with his car deals. Slasher moves to the back of the unit and stands against the back wall as the survivors tell him to be quiet. After looking around the unit one of the monsters sees a hole and looks through spotting Slasher's butt, the monster proceeds to rape him through a pipe, impregnating him. A monster then immediately bursts through Slasher's stomach and Slasher becomes Slasher/monster hybrid, killing the five unknown survivors in the unit. Biker Queen runs over to the unit and opens it, releasing the two biker girls and the Slasher monster (also the remains of the five unknown survivors). They run from the Slasher/Monster Hybrid, following Hobo who climbs down a hole, in an attempt to hide inside a buried school bus, which also happens to be his home/meth lab.
The monsters follow the 4 of them and somehow make it inside the buried school bus. During the fight one of the biker girls, Tit, has her eyes gouged out by a monster and her head savagely crushed. Biker Queen is finally able to get the bus started and as they leave, Tat, having no choice lights Tit Girl, Hobo and the monster on fire who then fall out the back of the bus. The bus emerges from underground with Biker Queen and Tat Girl having the intent of abandoning the remaining survivors but the bus finally dies in the street as the other survivors (Lightning, Secrets, Greg and Bartender) catch up and make it on the bus. the monsters immediately swarm the bus, they are saved by the mysterious Prophet, only known as Short Bus Gus, who seemingly has the ability to repel the monsters. He then leads the survivors into the sewers in an attempt to reach the big city. While working their way through the sewers Tat Girl encounters three infected townies, one of them known as "Puker Girl". Tat Girl is ultimately killed when Puker Girl stabs her belly repeatedly, with Biker Queen apparently saddened at her death, the survivors are about to be killed by the infected until another survivor known as Jean Claude Segal jumps down from the surface and saves the survivors by killing the infected (Puker Girl is seen fleeing further into the sewers). Jean Claude try to lead the survivors to the surface when he is attacked by a monster and has one of his arm bitten off leaving Jean Claude screaming in agony, the survivors are then separated into two groups (Jean Claude and Bartender) and (Biker Queen, Secrets, Greg, Lightning and Short Bus Gus). While separated Jean Claude is attacked again this time by an infected rat, suffering from a small but severe wound, Bartender tells him that the need to stop the bleeding and close the wound. So Bartender takes gunpowder from a bullet and pours all of it into Jean Claude's arm. Before lighting it Jean Claude asks "Did you learn this in the marines?" to which Bartender replies "No... Rambo 3" much to Claude's dismay. Bartender then lights the wound which blows off Jean Claude’s other arm, when Claude recovers Bartender tells Jean Claude that they have to kill as many infected as possible but Jean Claude protests stating that he has no arms to defend himself, Bartender takes the liberty to slap him a few times then Bartender replies "You only need legs to kick ass, baby boy!" as Bartender (with a reluctant Claude) charges towards a horde of the infected (using one of Claude's arms to defend himself also with).
This then leads to the other group of survivors finding the Hive, which is a gigantic rave with infected townies and monsters (whom spew their vomit on the people) which is what infects them causing horrible mutation and insanity. The survivors are reunited but are spotted by an infected townie similar to Puker Girl. Biker Queen gruesomely disembowels her, stating "that's how you gut a bitch" and almost soon after a monster vomits on her in the ensuing chaos,then all the survivors regroup and escape to a ladder. Jean Claude, unfortunately having no arms, volunteers to stay behind to fight off infected townies to give the survivors a chance to escape. Jean Claude manages to fend off the infected townies for a short while before being brutally ripped in half by a pair of monsters.
Short Bus Gus finds out that it has been his malfunctioning hearing aid that has been repelling the monsters and is impaled by a monster and repeatedly slammed against the wall. The Slasher monster finally catches up with the group and kills the attacking monster and then forcibly removes the pipe lodged in Greg's head, killing him instantly. Secrets is driven insane with grief and pounces on Slasher savagely attacking him, ripping pieces of his flesh off with her teeth as the remaining survivors join in on the assault. Ironically, Secrets uses the same pipe that was lodged in Greg's head (that Slasher had pulled out earlier) and hammers it in Slasher's head, killing him instantly.
In the final scenes of the film, we see the survivors tie a dead monster to the back of Biker Queen's newly acquired dirt bike, they cause the monster to start an ear piercing wail as Biker Queen takes off in an attempt to draw the monsters away from the remaining survivors (Secrets, Lighting and Bartender). We then see Bartender tell the other two that they are the only survivors left and that they have to repopulate starting now. Secrets looks up to see a giant robot foot come down and crushes both Secrets and Lightning.
The film ends with a mariachi singing about the "Feast" series, stating there might be a fourth installment answering many questions, but if this doesn't happen, "Don't blame me."
After the mariachi, the camera pans out to show Bartender slowly walk towards the camera, with a slight murmur, "Goddamn it."