Plot
The town of Dillford, Ohio, is home to three species who despise each other: humans, vampires, and zombies. Vampires are at the top of the food chain, both literally and in social circles, and tend to be the cooler kids in town. The zombies are able to leave their ghetto with special collars that keep them compliant. The humans languish somewhere in the middle, intimidated by the vampires while seeing the zombies as second-class citizens. All seem to have their own communities and rules by which they keep a tense truce.
School nerd Dag sets out to lose his virginity with Lorelei, the hottest girl in school to prove he isn't a loser. He has issues with Rick Wilson, the baseball coach and manager of the local meat factory. Rick fired all of the human employees, including Dag's mother, and replaced them with zombies as cheap labor. Rick taunts him with this as Dag practices his pitching and he chokes and ends up hitting one of the star athletes, injuring him severely.
Dag's hippy parents try to cheer him up with a speech about the men in his family being late bloomers. When the euphemisms about puberty and "huge changes" make Dag uncomfortable, he stops his well-meaning parents and tries to forget about the conversation.
Meanwhile, social outcast Petra is seduced and transformed into a vampire after being bitten by Milan - the high school's playboy vampire - while they are making out. Later he rejects her when she wants to make the relationship permanent, leaving her alone and now a vampire with no idea how to cope.
Dag's former best friend Ned, the brother of the athlete Dag injured, is also a social outcast at high school. He is one of the smartest people in the town but is ostracized by his sport-loving father and receives only contempt from his teachers. Following an argument with his family, Ned visits the zombie ghetto and he allows a female teenage zombie to bite his leg, transforming him into a zombie so he doesn't have to think or be smart anymore.
Just as Dag is finally about to score with Lorelei, out of nowhere aliens invade Dillford, causing residents to believe that the vampires, zombies, and humans are attacking each other. This causes an all-out war between the three groups that envelopes the town in a free for all of vampires vs. humans vs. zombies.
Dag and Lorelei hide in a house which is raided by a group of vampires and Lorelei is seemingly killed by Petra, who has always been jealous of her. In subsequent brawls between the factions, Dag, Ned, and Petra reluctantly join and hide in Stuart Miller's basement, Stuart being the local doomsday nut. While sheltering from the invasion, Petra and Ned are overcome by hunger and eat Stuart and his mother.
The aliens are able to determine their location, causing them to seek cover at the high school. Ned and Dag fight about their former friendship and Dag not wanting to be a loser anymore so they stop being friends. Ned, who is still a genius despite his non-verbal zombie status, leaves angrily. Later Petra and Dag become trapped and make out, but he flinches and Petra accuses him of being afraid of her. The aliens find them again, and Dag realizes that the aliens can only see inorganic materials, such as their synthetic clothing. The three then strip and run to Dag's house, where they change into organic cotton clothes kept by Dag's hippie mother.
Later, Dag, Petra, and Ned are captured by the aliens. They are placed with other captured residents in a warehouse, with force fields segregating the humans, vampires, and zombies. Dag and Petra find that instead of dying, Lorelei has been changed into a hot vampire, which intimidates Petra. After figuring out the weakness in the force field keeping them apart, Dag, Ned, and Petra convince everyone to band together to fight the aliens and destroy the force field.
At Rick Wilson's meat factory, the chemical "Tetrafluoraxipan" is used to transform horse cartilage into a meat-like substance and is what the aliens are after. Dag convinces Rick to let the town into the factory, as the aliens arrive and submerge in the chemical, combining to grow into a single large alien. It begins to make a speech about how they just wanted to ask nicely for the chemical, but the town's irritated residents interrupt when it starts getting so bad & haywires towards them on their own so they locked the citizens up to protect them from each other. Then the alien proceeds to quote Billy Joel's song to scold everyone for their behavior. But unfortunately Petra insults the alien for being condescending and points out it was them who scared everyone in the first place, including and inspiring Rick to throw a torque wrench at the creature. Furious, the single large alien begins attacking the crowd, hitting people with its huge hands. As everyone runs away, Keller gets smashed into blood drops and Petra is knocked back, accidentally dislodging an ammonia hose. The ammonia hose starts to melt the giant alien and Dag quickly gets people to grab more hoses and spray as much ammonia on the single large alien as possible.
Meanwhile, Milan corners Petra and attempts to kill her. Dag sees this and remembers Petra's words about hesitating, so he immediately jumps at Milan and starts a fight. Dag manages to throw a couple of punches, but Milan is too strong and quickly overpowers him. At that moment, the clouds move in the sky: revealing the full moon. The light touches Dag through the window and suddenly his body starts changing into a more hairy shape—it turns out Dag is a werewolf, and that’s the change his parents mentioned. He realizes this was the "big change" his parents were trying to talk to him about earlier. With his new powers, Dag fights Milan again in a fair match and Dag says to not touch. After exchanging a few hits, Milan comes after Dag with a knife, but Dag accidentally dodges just in time and Milan ends up impaled on a wooden spike, instantly exploding.
As the outclassed alien shrinks back to its mini size, it attempts to escape by spaceship while dropping a bomb to destroy the town. Ned catches the bomb, proving he's just as good as his athlete brother despite being a zombie. He gives it to Dag who pitches it into the ship before it closes its doors and the spaceship explodes, destroying the aliens.
Everything returns to normal for the town, and Lorelei asks Dag out on a date. However, Dag chooses to be with Petra instead. Ned appears to be happier and better adjusted as a zombie and seems to have bonded with his star athlete brother who has also become a zombie.
