The story begins on "the strangest day" of Larry Burrows (James Belushi) life consisting of a series of comic and dramatic misadventures. Larry, who blames all of his life's problems on the fact that he struck out during a key moment of a high school baseball game, wishes he had done things differently. His wish is granted by a guardian angel-like figure named Mike (Michael Caine), and appears at various times as a bartender, a cab driver, and so on. Larry soon discovers that Mike has transferred Larry into an alternate reality in which he had won the pivotal high school game. He now finds himself rich and (within his company) powerful, and married to the boss's (Bill McCutcheon) sexy daughter Cindy Jo Bumpers (Rene Russo). At first, his new life seems perfect, but he soon begins to miss his best friend Clip Metzger (Jon Lovitz) and wife Ellen (Linda Hamilton) from his previous life; he also discovers that his alternate self has created many enemies, like Jewel Jagger (Courteney Cox), and as Larry's problems multiply, he finds himself wishing to be put back into his old life.
The story begins with Larry's car, an old Ford LTD station wagon, stalled out in a dark alley. Suddenly the pink lights of the "Universal Joint," a bar, come on. Larry goes inside to call a tow truck, and tells bartender Mike his troubles. He reviews the day he just had, which ended with his getting fired after discovering his boss' (Niles Pender's)scheme to sell the company under the nose of its owners to a group of naive Japanese investors. He tells Mike that he wishes he'd hit that last pitch out of the park, after which Mike fixes him a drink called "Spilled Milk."
Larry leaves the bar, walks home and discovers someone else living his his house, which is now fixed up (previously his yard and driveway were muddy and unfinished). Mike appears as a cab driver and drives him to his "new" home, a mansion in Forest Hills, explaining that he did in fact hit the last pitch and won the game. He soon discovers that Cindy Jo is his wife and he's the president of his company, Liberty Republic Sporting Goods. Being a classic car buff, he's shocked to find that he owns a collection of priceless antique automobiles.
He soon discovers that Clip has a low-level job in the accounting department, and Ellen is shop steward (in both realities). Jewel Jagger is now Larry's mistress. Ellen hates Larry and he discovers that the union is threatening a walkout due to massive layoffs and increased production, since Niles Pender is selling the company in both realities. Seeing Ellen, he realizes how much he misses her and agrees to all the union's demands, providing Ellen agrees to dinner at his favorite restaurant. She reluctantly agrees, and Larry eventually convinces her that they were married in a previous life.
Pender, after discovering that Larry has agreed to union demands, takes revenge on Larry, by telling both Cindy Jo and Jewel, of Larry's dinner date with Ellen. He then plots to kill Larry at the office that night. However, Leo Hansen, Liberty Republic's owner, comes to Larry's office to leave him a note that he is fired for cheating on Cindy Jo, and Pender, thinking it's Larry, kills him. He then calls the police, who attempt to arrest Larry for Leo's murder. Larry escapes while jealous Jewel creates pandimonium outside in her attempts to shoot him (and shoots out a number of police cars in the process), a police chase ensues, and Larry is eventually cornered in a dark alley. As he's about to turn himself in, the pink glow of the "Universal Joint" comes on, and Larry runs into the empty bar, attempting to remake the "Spilled Milk" drink that Mike made before.
The flashing lights of the police cars become those of a tow truck, Mike's back behind the bar, and Larry realizes he's back in his old life. He suddenly realizes that tonight the owners of Liberty Republic are to sign the deal with the Japanese investors, rushes to the company headquarters with Duncan in the tow truck, and exposes Pender's scheme just as Leo is about to sign the deal.
Jackie Earl (Sanders), company president and Cindy Jo's husband, offers Pender's job to Larry, plus a company car, a new Mercedes. Larry accepts the offer and the film ends happily with Larry's 35th birthday party at his home, glad to be back with Ellen, and in his house, which still has the muddy driveway and lawn.
- James Belushi — Larry Joseph Burrows: The film's protagonist. A normal guy with an ordinary life who wishes it to be different, and is granted that wish and becomes president of the company with a big house.
- Linda Hamilton — Ellen Jane Burrows/Robertson: Larry's loving wife in the normal reality. She is the head of the labor union. In the alternate reality, she looks at Larry with much disdain.
- Michael Caine — Mike the Bartender/Mr. Destiny: A bartender and guardian angel like character who is responsible for showing Larry what his life would be like if things had been different.
- Jon Lovitz — Clip Metzler: Larry's lifelong best friend. Is shown to be a prankster and a goof-off. In the alternate reality, Clip is suffering from low self-esteem and is suicidal.
- Hart Bochner — Niles Pender: The film's antagonist. The head of Larry's department at work, who schemes and plots to take over the company.
- Bill McCutcheon — Leo Hansen: The owner of the company Larry works for, an old but very nice man. He is shown to be a bit absent minded at times.
- Rene Russo — Cindy Jo Bumpers/Burrows: Leo Hansen's attractive daughter. In the alternate reality, she is Larry's wife.
- Jay O. Sanders — Jackie Earle Bumpers, a.k.a. Cement Head: An ex-football player who is married to Cindy Jo and is president of the company Larry works for. He is neither seen nor mentioned in the alternate reality.
- Maury Chaykin — Guzelman: A contractor who is reluctant to finish paving Larry and Ellen's driveway.
- Pat Corley — Harry Burrows: Larry's father who works in the warehouse of the company Larry works for. In the normal reality he is shown to be totally faithful to Larry's mother. In the alternate reality, it is mentioned that he is divorced from Larry's mother and dates younger women.
- Douglas Seale — Boswell: Larry's personal servant in the alternate reality.
- Courteney Cox — Jewel Jagger: The forklift operator in the normal reality. Is shown to have a dark side which becomes more evident in the alternate reality, with whom Larry finds out he has been having an affair with.
- Doug Barron — Lewis Flick: Accomplice to Niles Pender, though he is shown to not be quite as cold-hearted as Niles.
- Jeff Weiss — Ludwig: Larry's chauffeur in the alternate reality.
- Jeffrey Pillars - Duncan: Tow Truck driver.
Portions of the film were filmed in Winston-Salem, North Carolina, using the baseball team from Richard J. Reynolds High School. The office building is the former headquarters of the R. J. Reynolds Tobacco Company.