The story begins on "the strangest day" of Larry Burrows (James Belushi) life consisting of a series of comic and dramatic misadventures.Larry is a businessman having just turned 35. He lives in an incomplete small house in a suburb of a nameless US city with his loyal and supportive wife Ellen (Linda Hamilton). Larry looks forward to having a romantic dinner with Ellen to celebrate his birthday that night, but she tells him that she may not make it due to her union meeting.Larry blames all of his life's problems on the fact that he struck out during a key moment of a high school baseball game 20 years earlier, which cost his team from winning and left him an outcast among his friends and teammates. Every day, Larry wishes he had done things differently and hit that baseball. Things in Larry's life turn from bad to worse when he gets fired by his reprehensible supervisor Niles Pender (Hart Bochner) after discovering a scheme to sell the company under the nose of its owners to a group of naive Japanese investors.As the defeated Larry leaves the office after cleaning out his desk, he drives home through a rough part of the city when his car, an old Ford LTD station wagon, stalls 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 (Michael Caine) his troubles. He reviews the day he just had to Mike and tells him 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 in his house, which is now fixed up (previously his yard and driveway were muddy and unfinished). Mike appears as a taxi 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. Mike more or less tells Larry that he is a type of 'guardian angel' known at Mr. Destiny who grants life-altering wishes to those select few whom wonder what their lives would be like if they made a different life-altering choice.After being dropped off, Larry soon discovers that Cindy Jo (Renee Russo), the daughter of the owner Leo Hanson (Bill McCutcheon), 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 in his garage.At first, Larry's new life seems perfect with the large birthday party that Cindy Jo throws for him, and happy that he has two young children from his marriage (something that he and Ellen could not have). But soon, Larry begins to miss his best friend Clip Metzger (Jon Lovitz) and wife Ellen from his previous life; he also discovers that his alternate self has created many enemies, like Jewel Jagger (Courteney Cox), a forklift driver at a company warehouse whom is angry with him over spending less time with her.Larry's problems multiply as he realizes that hitting that home run not only made him popular and well liked, but it has also made himself arrogant and shallow. He soon finds himself wishing to be put back into his old life. Larry 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 (because in this alternate reality he has become greedy and selfish) 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. He first tells 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 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 by beating him with a golf club. As a 'Plan B', Pender then calls Larry to the office, and then calls the police where he points out to them that he witnessed Larry killing Leo.As the police attempt to arrest Larry for Leo's murder, Larry escapes while the jealous and insane Jewel creates pandimonium outside the building in her attempts to shoot him (and shoots out a number of police cars in the process), and a police chase ensues.Larry attempts to hide out at Ellen's house, but she refuses to help him and instead tells Larry that she has gotten back together with her husband. Dejected, Larry leaves. Now a fugitive framed for Leo's murder, Larry finds himself cornered in a dark alley. As he's about to turn himself in, the pink glow of the "Universal Joint" bar comes on, and Larry runs into the empty bar, attempting to remake the "Spilled Milk" drink that Mike made before and wishing that he had his old life back and wishing he never hit that home run.The red flashing lights of the police cars outside the bar suddenly become those yellow lights of a tow truck. Suddenly, Mike appears back behind the bar, and Larry realizes he's back in his old life at the exact same time and day of his 35th birthday. He suddenly remembers that tonight the owners of Liberty Republic are to sign the deal with the Japanese investors. He has Duncan, the tow truck driver, rush him back to the company headquarters and exposes Pender's scheme just as Leo is about to sign the deal. Larry then gets a personal revenge by knocking out Pender with one punch before leaving.Larry then returns home to his house where, after walking in, Ellen is revealed to be there with all of his neighbors and friends where they throw Larry a suprise birthday party. Chip is also in attenance with Jewel as his date. Just then Jackie Earl (Jay O. Sanders), the company vice president and Cindy Jo's husband, arrives and tells Larry that Pender confessed everything to Leo which resulted in his dismissal and that Leo is offering 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.Larry then recalls one final flashback where 20 years earlier, after striking out and losing the game for his team and being all alone on the baseball diamond field, he remembers a strange man (whom is Mike) saying to him that all things in life will turn out for the better or worse.