Paul Bland is a balding wine snob who works at a cheap wine shop, and his attractive wife Mary is a nurse who is routinely groped by hospital patients. They bemoan their status, and dream of opening a restaurant. A loving but exceptionally prudish couple, they sleep in separate beds and disapprove of sex, except for "a little hugging and kissing". When Paul is fired, they are left with barely enough money to get by, and despair that they will ever realize their dream. Their plight is exacerbated by the fact that they live in an apartment building that is a regular site of swinger parties, which they despise.
When a drunk swinger wanders into their apartment and tries to rape Mary, Paul hits him on the head with a cast-iron frying pan, unintentionally killing him. Figuring that no one will miss him, they take his money and put his body in the trash incinerator. Later, they kill another swinger in a similar fashion, and realize that they could make money by killing "rich perverts". They get advice on infiltrating the swinger community from one of the building's orgy regulars, Doris the Dominatrix. Mary lures men by promising to satisfy their sexual fetishes, and when they try to have sex with her, the otherwise timid Paul becomes alarmed enough to kill them with the frying pan. The Blands are surprised to learn how efficient their scheme is.
After finding a flyer for cheap lock-installation service on their car, they decide – for the safety of Paul's wine collection – to have a new lock installed on their apartment. The locksmith is Raoul Mendoza, who uses the service as a scam which enables him to commit burglary of his customers. Using his own key, he sneaks into the Blands' apartment the following night, and stumbles across the corpse of the Blands' latest victim. Paul confronts Raoul, and with each in a compromising position, they strike a deal: neither will report the other, and Raoul will dispose of the bodies, which he says he can "exchange" for cash for them to split.
One night, Mary's client does not show up, so Paul leaves to buy groceries (and a new frying pan, since Mary is "a bit squeamish about cooking with the one we use to kill people"), leaving her alone. However the client arrives late, refuses to accept Mary's protest that he missed his appointment, and tries to rape her. Raoul happens to arrive, and strangles the client. Raoul then offers Mary marijuana and they have sex. Paul subsequently becomes suspicious of Raoul and follows him, discovering that he has been selling the bodies to a dog food company, and – unbeknownst to Paul and Mary – selling the victims' expensive cars and keeping the money.
Raoul and Mary have sex again, and he tries to persuade her to run away with him. He tries to run Paul over with a car, who responds by hiring Doris the Dominatrix to use her costuming and role-playing experience help get rid of him. She poses as an immigration agent threatening Raoul – a Mexican immigrant – with deportation, and a public health worker who gives him pills that are secretly saltpeter, to render him sexually impotent. These schemes do not work.
The Blands are in danger of losing the property they wish to buy for the restaurant to another buyer. To drum up business quickly, they attend a swingers party in search of victims. Paul loses his temper and hurls a space heater into the hot tub, killing all of the partygoers and enabling Paul and Mary to take all their money and sell the cars themselves.
A drunken Raoul breaks into the Blands' apartment and threatens to kill Paul for not sharing the money from the cars, and because he wants Mary for himself. He informs Paul that he and Mary will be getting married, and tells Mary to bring a frying pan to kill Paul. Instead, Mary lures Raoul into the kitchen and kills him. Mary and Paul remember that their real estate agent (who is helping them buy the restaurant) is due to arrive soon for dinner. With no meat in the house and no time to buy any, they cook Raoul and serve him for dinner, describing the dish – which the agent suggests they include on the menu – as "Spanish". The last shot of the film is Paul and Mary smiling in front of their new restaurant, with the caption, "Bon Appétit".
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