The film centers around Thane Furrows, who spends the day messing around his apartment and complaining about a number of random subjects, among other things, flies, popsicles, junk mail, his boss' wife, his upstairs neighbor, smoking, salesmen, and philosophizes on a number of things such as the morality of eating humans and the sensibility of keeping pets. A number of minor annoyances also perturb him throughout the day: a fly lands on his cereal at breakfast, which he inadvertently eats; an insurance salesman named Ray comes to the door, to which Furrows responds by feigning interest and, shortly after promising to take out a number of policies, slams the door in Ray's face with the words "I'd rather be dead"; an automated survey about carpet cleaning calls him repeatedly; his boss' wife comes by to pick up a book he was writing, and he (eventually) tells her off.

Furrows has a number of strange philosophies: he wishes his children's books to be instructive for the good of society, such as "How to Start the Family Car" (in case "someone chokes on a chicken bone" and "there are no adults around"), and "Bye Bye Grandma" which he wants to help accustom children to death. He refuses to keep pets because he feels they would "turn on you" in a food shortage, choosing instead to keep a cardboard cutout of a dog named Pete.

After the fly incident, Furrows suffers from a number of scares. When closing his eyes, he repeatedly sees a menacing face. He receives numerous messages from phone and mail about "eight o'clock".

Furrows' only friend appears to be a man named Al, who comes by in the afternoon for a visit. They eat cereal and Al tries to dissuade Thane of his cynicism. While Thane attacks the optimism of people like Al, he seems comforted by Al's sympathy over the visit of Melanie, the boss' wife.

Later that night, Furrows loses an arm wrestling match to the noisy neighbour upstairs, thereby giving him the right to play metal as loud as he wants whenever he wants. After a day of "messing around", Furrows receives a knock at the door at the dreaded "eight o'clock" and is greeted by a limo driver. When he steps into the limo, the driver (Jim Carrey) turns around and reveals himself to be Death. Death tells Furrows that he has met his quota of saying "I wish I were dead" and must die, and Furrows complains about the stupidity of the rule until Death, unable to scare Furrows into line, puts him back into his body. Furrows awakes with frightened Al standing over him, trying to wake him. The movie ends with the two going out to a restaurant, though Furrows' insistence that they serve him cereal suggesting the lessons of the past day were lost on him.

After the credits, a short epilogue involves Death stopping the limo in a dark space and looking at the heavens. He claims that he just couldn't stand Thane and had to return him to life. He adds that he isn't ever coming back for Thane, either, implying that Thane may have just accidentally become immortal.