The film opens with a missing girl (Elizabeth Moss) poster being pecked at with a crow during a storm. After the opening credits, a young woman named Paulie (Annabeth Gish) is seen cutting a tomato from the garden before going back inside where she is preparing food for her three friends waiting for her on the sofa. They are watching a documentary interview of Dr. Arbuthnot (Ron Perlman) when a disagreement ensues over the Doctor's strong anti-gay remarks. Just then the front door opens and Pete (Ron Eldard) appears. He has had car trouble in the storm and has been given a lift by a passing stranger (Bill Paxton). Marc (Jonathan Penner) invites him in to join them for dinner.
During dinner, the stranger introduces himself as Zack. The five guests introduce themselves as Pauline, Jude (Cameron Diaz), Pete, Marc, and Luke (Courtney B. Vance). They are liberal graduate college students who live together in this rustic house somewhere in rural Iowa. But the dinner soon turns unpleasant when Zack turns out to be a racist, anti-Semitic Desert Storm veteran who supports Nazism. Haunted by his possible post traumatic stress disorder, and after a tense political debate that includes Zack making statements such as "Adolf Hitler had the right idea" and "the Holocaust can't be proven," the evening takes a turn for the worse, with Zack threatening the group, putting a knife to Marc's throat and breaking Pete's arm after Pete tries to threaten him. Marc kills Zack by stabbing him in the back, and the group decides to cover up the murder.
After a long discussion led by Luke, the students decide to continue killing people with conservative views, in order to make the world a better place. The initial horror of the event soon turns to intrigue amongst the housemates who have numerous discussions on the implications of what they have done whilst debating different views of their actions of many bottles of wine and digressing to problems of the world and politics. They discuss different methods of murder and settle on poison as being their first method used for the next case after Zack.
The students lay down a procedure for each murder. The guest will be given every opportunity to change their mind and recant their beliefs. If the guests fail to change their ways by dessert, the group offers them poisoned white wine from a blue decanter and raises a toast. The bodies are buried in the group's vegetable garden.
The following night, the group invites the local priest (Charles Durning) for dinner and lace a blue wine carafe with poison and reserve it for the priest's glass, while the green wine carafe is for them. Their previous discussions on good versus evil and man's right to serve judgement are to be played out. The priest's strong views on homosexuality and AIDS only serve to villify the housemates and justify their intended action. The priest eats his fill at the table and drinks deeply before beginning to feel ill from the effects of the poison. The housemates look on in mock horror and fascination whilst the priest breathes his last.
Paulie feels guilty about what they have done, but the others console her and justify it based on the priest's strong opinions on certain topics that disagree with the views of the housemates, who go on to finish dinner with the priest still slumped at the top of the table. Suddenly, Marc begins to cough and act as though he is poisoned too, but it turns out to be a joke on the other housemates.
Their next dinner guest is a man (Mark Harmon) with strong sexist opinions who considers rape to be a false state of mind and thinks that men are entitled to their position of superiority. The guest drinks from the poisoned blue chalice and soon begins to feel the effects to the amusement of the housemates, before he is buried in the garden alongside the priest.
The film cuts to door-to-door enquiries being made by a local sheriff Alice Stewart, (Nora Dunn) about a missing girl Jenny Tyler, the subject of the poster at the start of the film.
The next guest/victim to meet her maker is a woman (Rachel Chagall), a pro-life extremist with strong views on abortion.
Meanwhile, Pete's abandoned car is found in the nearby woods by the sheriff who notes the licence number and contacts him to make enquiries about a gun found in the back seat.
The next dinner guests are a pro-Muslim activist (Warren Hutcherson) and then an Homeless basher (Nicholas Sadler) - meanwhile the tomatoes are blossoming in the garden as they benefit from the 'added nutrients' and Jude takes to caring for the garden, and Paulie makes chutney from the fruit.
A environmentalist is the next victim (Jason Alexander) who, after sharing his strong opinions on global warming drinks from the poisoned blue bottle and the results are inevitable. Following that is a Neo-Nazi (Rick Lawrence), and then an illegal alien hater (Amy Hill).
Meanwhile, the sheriff is stepping up her inquiries and reveals to one day Pete that she has also found Zack's abandoned pick-up truck in the woods. Zack is a known felon and Pete feigns shock when she tells him that Zack is a suspected serial killer, but Pete denies knowing who Zack is and does not identify him from a series of photographs shown to him by the sheriff.
The housemates discuss their progress and the next guest, a librarian (Pamela Gien) is invited to dinner. But the meal is interrupted with a knock at the door, the sheriff has made a house call and gets Marc to reluctantly open the door. She is asking about the missing girl, named Jenny Tyler, and shows Marc the same photographs that she had earlier shown Pete. Paulie joins them at the door, flicks through the photographs and denies knowing any of them even though the picture of Zack is included. While they are at the door, there is a crash in the background that the sheriff hears (presumed to be the librarian collapsing). Pete rushes to the door to cover and meets the sheriff again. They make small talk to distract the sheriff who eventually leaves, but pauses in the pathway, suspicious on the students nervous behavior .
When the housemates return to the dinner table, they find Jude in a panic because she has had to stab the librarian because she was a tee-totaller and didn't drink the wine. A discussion ensues where the housemates try to justify their killing of Zack given his criminal past, with them declaring that they have prevented other deaths by killing him. They then argue over their body count to date and excuse their actions based on their opinions of their victims and their strong views, declaring that they have saved society.
After ten murders, misgivings begin to surface within the group as a couple of them grow indecisive regarding the justification of their actions. Their next dinner party guest is a young teenage girl called Heather (Bryn Erin) who has strong opinions for her age on her human rights and family values, and an opponent of mandatory sex education in school. Her opinions quickly anger the housemates and Luke takes a particular dislike to her, with the other housemates arguing amongst themselves. Jude eventually comes to Heather's rescue and accompanies her out of the house and she makes a lucky escape.
The next day, the sheriff secretly arrives outside the students' house and peers through the garden fence. She overbears Jude and Luke argue over the tending of the garden and tomato plants and tensions are high.The sheriff scales the fence to get a closer look when she thinks that Jude and Luke have gone back inside, but gets caught creeping around the garden by Luke who kills her with a blow to the head with a shovel.
A few days later, Pete and Luke head to the airport for Spring Break but their flight is delayed. By coidence, they see Dr. Norman Arbuthnot in the departure lounge and approach him to introduce themselves. (Throughout the movie, brief segments of radical statements made by Arbuthnot had been appearing on the TV that the group had been watching in their home.) They invite him back to their house for dinner and prepare the intended blue bottle poisoned solution, but allow him to drink from an alternative green bottle at his own selection for the time being.
They go on to debate his views and Arbuthnot reveals that his extreme conservative views he shares on television are about gathering high ratings rather than necessarily being his own personal opinions. His views turn out to be moderate and reasonable, much to the suprise of his five guests. Arbuthnot reveals that he is actually a full-on liberal and he stymies the group with his moderate and persuasive arguments, all of which the usually argumentative group have difficulty debunking. He even admits that he says more radically conservative things mostly for attention. When he reaches for the blue carafe, Jude prevents him from drinking from it saying "It was left out too long and has gone bad," much to Luke's horror. They continue their debate.
After several more minutes of talk, Arbuthnot's responses to the issues of abortion, racisim, sex-ed, and women are unbias and agreeable. Out of pure frustration, all five confused students one by one suspiciously excuse themselves to the kitchen to determine Norman Arbuthnot's fate, leaving the behildered speaker activist alone at the dinner table. In their absence, Arbuthnot goes to fill his glass from the blue bottle but he smells something strange and sets the blue bottle aside thinking that the wine is fermented. (Note: prior to him arriving, an overzealous Marc has made the solution too strong with too much arcenic which would explain an overpowering smell the wine now gives off.)
In the kitchen, the five housemates talk over what they should do with Arbuthnot since he is really a fair-minded liberal like them. A quick vote is taken on whether they should kill Dr. Arbuthnot, and it's 4 to 1 to let him live. But Luke continues to hold out, instisting that he must be killed and even calls Arbuthnot "Hitler" for he is a skilled manipulator. After a tense altercation, Luke pulls out a gun and aims it at Jude. Luke confesses to killing the sheriff a few days ago, who until now, the other housemates did not know had been killed in their back yard.
Back in the dining room, a curious Dr. Arbuthnot looks around at the various things in the dining room. After a few minutes, he casually takes the time to light himself a cigar when he spots the ten freshly dug mounds in the rainy back garden when the lights in the room flicker for a few seconds from the storm outside. Arbuthnot then picks up and reads a discarded newspaper lying on the couch which has a headling on the missing sheriff. Norman Arbuthnot looks towards the strong smelling blue bottle, and then back at the ten mounds of dirt outside in the pouring rain, and then back at the blue bottle... he begins to put two and two together.
In the kitchen, Jude dials 911 and Luke points the gun at her, but Jude is placed on hold by the police department so she hangs up. Luke breaks down and the house mates console him before rejoining Dr. Norman Arbuthnot in the dining room. Arbuthnot presents the group with glasses of wine and offers them a toast but does not drink himself, with the excuse that he doesn't want to be too intoxicated to fly his private plane.
The film closes with the image of a painting showing five dead dinner party hosts and the voice of the Dr. Arbuthnot, the presumption being that he had filled their glasses from the blue bottle after realizing their deeds and delivering his own form of justice as delivered in a final voice-over semon to his listeners.