As people are enjoying drinks in a bar, a man covered in blood (identified onscreen as "Hero") enters through the door and warns them all of impending danger. No one heeds his warning, so he shows the bar patrons the head of a repulsive monster to make them take him seriously. He is soon pulled through a window and decapitated by one of the monsters, amusingly just after it was implied that his chances of survival were very high.
After the carnage, a woman ("Heroine") bursts through the door and reveals herself to be the recently deceased man's wife. After a brief sentimental moment between the wife and her late husband, they begin boarding up the windows in the bar. Despite their efforts, a young monster bursts through an uncovered window and begins decapitating the people inside the bar. It severs the leg of one of the women ("Harley Mom"), and it is initially assumed that she died from massive blood loss.
The monster disappears for some time, then is found attempting to sexually penetrate one of the deer heads nailed to the wall. A shotgun blast removes the deer head and monster from the wall. The monster drops into a freezer which is then sealed shut, trapping it inside.
Following this, the remaining windows are boarded up and the bar patrons are given a moment of peace. Trying to call for help, they learn that the only phone in the bar has been hit by a stray shotgun blast and has been rendered useless. After a short breather, one of the women ("Tuffy") suddenly realizes that her son is still upstairs and runs to get him.
Once she finds her child the group rejoices until the boy is pulled through a window and eaten by one of the monsters, leaving only his sneaker behind. Tuffy is now incapacitated by grief, and the monster then vomits a stream of slime at one of the group ("Beer Guy"). As the remaining people regroup downstairs, they realize that the slime has a decomposing effect and that the victim is being slowly overcome by its effects.
The group kills the young monster in the freezer and hangs it outside. The monster's parents quickly have sex and produce two offspring in a matter of seconds, all of whom begin to attack the pub with renewed fury. Meanwhile, one of the women ("Honey Pie") begins washing off the blood and has to take off her clothes, much to the amusement of the others. The patrons regroup and enact various attempts to escape or drive off the monsters, all of which lead to more casualties, including the accidental death of the Heroine at the hands of another character (the aptly nicknamed "Bozo"). Driven by rage over the death of her child, Tuffy aggressively takes charge of the remaining survivors, which results in the audience seeing her nickname change from "Tuffy" to "Heroine 2". "Honey Pie" successfully makes it to a truck, giving the other characters brief cause for hope (until they realize she's speeding off by herself).
After many attacks and ultimately, a fight to the death between the last remaining humans and monsters, only four people survive the ordeal: Bozo, Hot Wheels, Tuffy (Heroine 2), and Honey Pie. One person ("Grandma") seems to survive but is seen being attacked by one of the remaining monsters at the end of the film.