William "Bill" Foster is stuck in Los Angeles traffic on a hot day. After his air conditioning fails, he abandons his car and begins walking, carrying his briefcase.

At a convenience store, the Korean owner refuses to give change for a telephone call. Foster begins ranting about the high prices. The owner grabs a baseball bat and demands Foster leave. Foster takes the bat and destroys much of the merchandise before leaving. Later, while resting on a hill, he is accosted by two gang members, who threaten him with a knife and demand his briefcase. Foster attacks them with the bat and takes their knife.

The gang members, now in a car with two friends, find Foster using a payphone. They open fire, hitting several bystanders but not Foster. The driver crashes. Foster picks up a gun, shoots the one surviving gang member in the leg, and then leaves with their bag of weapons. Foster encounters a panhandler and gives him the briefcase, which only contains his lunch.

At a fast food restaurant, Foster attempts to order breakfast, but is told they have switched to the lunch menu. After an argument with the manager, Foster pulls a gun and fires into the ceiling accidentally. After trying to reassure the frightened employees and customers, he orders lunch, but is annoyed when the burger looks nothing like the one on the menu. He leaves and tries to place a call from a phone booth, then shoots the booth to pieces after being hassled by someone who was waiting to use the phone. After Foster calls "home" again and states his intention to attend his daughter's birthday party, his ex-wife Beth notifies the police because she has a restraining order against him.

Sergeant Martin Prendergast, who is on his last day of duty before retirement, insists on investigating the events. Interviews with witnesses lead Prendergast to suspect that the same person is responsible for all of them. Foster's vanity license plate proves to be an important lead, because Prendergast remembers being in the same traffic jam as Foster earlier that day. Prendergast and his partner, Detective Sandra Torres, visit Foster's mother, who is surprised to learn that he lost his job. They realize Foster is heading toward his former family's home in Venice and rush to intercept him.

Foster passes a bank where a black man is protesting after being rejected for a loan. The man exchanges a glance with Foster and says, "Don't forget me," as he is escorted away by police. Foster stops at a military surplus store to buy boots. The owner, a homophobic Neo-Nazi, diverts Torres when she comes in. After Torres leaves, the owner offers Foster a rocket launcher and congratulates him for shooting "a bunch of niggers" at the restaurant. When Foster expresses distaste for the store owner's bigotry, the man becomes hostile and attempts to turn him over to the police, but Foster stabs him then shoots him dead. Foster changes into army clothes, takes the rocket launcher and leaves.

Foster encounters a road repair crew who are not working and accuses them of doing unnecessary repairs to justify their budget. He pulls out the rocket launcher but struggles to use it, until a boy explains how it works. Foster accidentally fires the launcher, blowing up the construction site. By the time Foster reaches Beth's house, she has already fled with their daughter. He realizes that they may have gone to nearby Venice Pier, but Prendergast and Torres arrive before he can pursue them. Foster shoots Torres, injuring her, and flees with Prendergast in pursuit.
At the pier, Foster confronts his ex-wife and daughter. Adele is happy to see him, but Beth is frightened. Prendergast arrives and distracts Foster long enough for Beth to throw his gun into the ocean. Prendergast holds Foster at gunpoint, insisting that he give himself up and acknowledges his complaints about being ill-treated by society, but does not accept that as an excuse for his rampage. With nothing left for him, Foster tricks Prendergast into shooting him dead so Adele can collect his life insurance. Having asserted himself, Prendergast decides to hold off retirement.