The film opens as a fleet of helicopters sprays for medflies, revealing all the characters along the path of their flight. Dr. Ralph Wyman and his wife, Marian, meet another couple, Stuart and Claire Kane, at Zoe Trainer's cello concert and make a spontaneous Sunday dinner date. Marian's sister Sherri is married to philandering cop Gene, who makes up unbelievable stories to hide his affair with Betty Weathers.
Betty is in the process of divorcing one of the helicopter pilots, Stormy. Waitress Doreen Piggot is married to an alcoholic limo driver named Earl. TV commentator Howard Finnigan lives with his wife Anne and their young family next door to Zoe and her mother, cabaret singer Tess Trainer. Their pool cleaner is Jerry Kaiser, whose wife, Lois, works from home as a phone sex operator, tending to the children while she talks off strange men. Jerry and Lois are friends with Doreen's daughter, Honey and her husband Bill, who works as a makeup artist. The day before Casey Finnigan's 9th birthday, Doreen hits him with her car.
Casey appears fine, and refuses Doreen's offer of a ride home, because she is a stranger. His mother comes home from ordering his birthday cake to find Casey slumped lethargically on the couch. Howard convinces her to take Casey to the hospital, where he remains unconscious. The baker calls the next day to inform Ann that the cake is ready, but Howard, wanting to keep the line free, briskly ends the conversation. The baker immediately calls back, incensed at being hung up on.
While the Finnigans maintain their vigil, the baker continues to call and harass the couple. Howard's estranged father turns up at the hospital and recalls that Casey's hospitalization is reminiscent of the day that Howard had an accident as a boy. When Howard's mother went to her sister's house, she found her in bed with Howard's father, whom her sister had seduced. That lead to the estrangement between father and son. Stuart and his two buddies, Gordon and Vern, harass Doreen at the diner before they head out on their three-day fishing trip. On the first day, they find a young woman's dead body submerged near some rocks. After some debate, they decide to tie her to the rocks, continue fishing, and report the body when they are done.
When he comes home, he eventually admits what they had done to Claire, and she is disgusted that they could fish for days with the woman's body nearby. The body is identified as a 23-year old woman, and Claire goes to her funeral out of a sense of guilt. Stormy visits Betty's house, ostensibly to pick up his mother's clock, but instead, he spends the day destroying Betty's belongings. Bill and Honey entertain themselves in the apartment that they are watching while its owners are on vacation by taking some pictures of Honey where Bill has made her up to look like she has been brutally beaten. Gene abandons the family dog on a strange street because he cannot endure its barking, but after several days of his distraught children's inquiries, he returns to the neighborhood and retrieves the dog. The Wymans get into a massive argument just before their dinner party with the Kanes. Marian admits to sleeping with another man. Both couples alleviate their stress by drinking copiously, and the party lasts all night long.
One day, Casey's eyes begin to flutter. Ann's excitement grows, but just as he appears to be fully waking, he suddenly dies. Seeing this and being overwhelmed, Howard's father, Paul Finnigan, leaves the hospital while the distraught couple returns home and informs Zoe of Casey's death. The next day, they go to the bakery to shame the baker over his abuse of them. When he learns why they never picked up the cake, he asks them to stay and gives them baked goods. Zoe, worn to the breaking point by her mother's alcoholism and her isolation, commits suicide by starting the car engine inside the garage and getting asphyxiated while she's playing the cello. Later that day, her mother discovers that Zoe's dead and is bewildered. When Honey picks up the pictures from the developer, they are mixed up with Gordon's.
Gordon is horrified to see the pictures of Honey beaten so badly, while she is horrified of the pictures he took of the submerged body on his fishing trip. They each walk away from each other memorizing the other person's license plates. Honey and Bill are on their way to a picnic with Jerry and Lois. In the park, Jerry and Bill try to hook up with two young women they encountered earlier, and Bill quickly makes an excuse to divvy up into couples. As he and one of the girls walk away from Jerry and the other, they hear her scream. They turn around to see Jerry hitting her in the head with a rock, just as a major earthquake strikes. The film ends as all the characters are shown during the earthquake. In the aftermath, Jerry's murder of the girl is attributed to a falling rock during the earthquake, and the implication is that he may also be responsible for the submerged body in the river.