Montgomery "Monty" Brewster (Richard Pryor) is a second-rate minor league baseball pitcher with the Hackensack Bulls. One night after winning a game, Monty and his best friend, the Bulls' catcher, Spike Nolan (John Candy), are hitting on two women at a local bar. When they try to leave with them, they're confronted by two large men who claim they are the women's husbands. Monty and Spike are punched out by the men and a large bar brawl starts. They are later arrested and thrown in jail. Their coach, Charley, tells them that management has decided to kick them off the team. In court, Monty tries to explain the fight to the judge, who is wholly unsympathetic. The photographer that had been following Monty suddenly rises and tells the judge that he represents an unidentified party that will post bail for both Monty and Spike if they plead guilty.
The next morning, they are fired from the team and have to sit in jail because they can't afford bail. A stranger named J.B. Donaldo (Joe Grifasi) (a lawyer posing as a photographer) offers bail and asks them to come to New York City with him. At the Manhattan law office of Granville & Baxter, Brewster is told that his recently deceased great-uncle Rupert Horn (Hume Cronyn), whom he has never met, has left him his entire fortune because Monty is his only living blood relative (by way of Monty's grandmother, Rupert's half-sister), but with several conditions.
Rupert's last will is on film and features the wheezy and ill-tempered Rupert himself. He relates a story from his youth where his father made him smoke a whole box of cigars to punish him for smoking just one. Rupert tells Monty that he wants Monty to hate spending money. Monty will be given $30 million to spend in 30 days. If he's successful, he'll be awarded the full inheritance of $300 million. Brewster is challenged to either take $1 million upfront (as part of a 'Wimp' clause) or spend $30 million within 30 days to inherit $300 million. If he chooses the former, the law firm becomes executor of the estate and divides the money among charities (after taking a sizable fee). In the latter case, after 30 days, he must spend the entire $30 million within one month under the following rules:
1.) With the exception of those who witnessed the will reading, whom are the firm's two senior partners George Granville (David White) and Norris Baxter (Jerome Dempsey) and Rupert's estate lawyer Edward Roundfield (Pat Hingle), Brewster cannot reveal to anyone the will's terms. He may only tell everyone else he inherited $30 million.
2.) Brewster must spend the money on tangible items. If anything he buys accrues value, such as an investment that earns money, that is considered part of the money he inherited and he must spend that as well.
3.) Directly giving away money is capped at 10%, split between 5% in gambling losses and another 5% maximum to be donated to charity.
4.) Brewster may not willfully damage anything he buys with the money.
5.) After 30 days, he may not own any assets that are not already his (in other words, nothing but "the clothes on his back").
If any of these rules are violated, the challenge is forfeited, all money that he has left goes to the law firm and Brewster will be left with nothing. Despite the tempting offer just to "wimp out" and walk away with $1 million, especially after the law firm reveals that they already did a background check on him and showed that the highest annual salary he ever earned was $11,000-a-year while a member of the Toledo Mud Hens, Monty decides to take the $30 million challenge.
The next day, Monty goes to a bank across the street where the money is waiting for him. He appoints Spike as the vice president of an investment corporation he's building and hires one of the bank's security guards as his personal guard for the next month. He also asks to rent the bank vault where the cash is stored and is granted a 30-day lease and refuses an account that will earn him interest.
On the street, Monty hires a Russian taxi driver (Yakov Smirnov) to be his personal driver. He has the driver rent a fleet of limousines and other drivers. He takes everyone in earshot out for an expensive dinner in Manhattan. At dinner he talks with the accountant that Granville and Baxter have assigned to him, Angela Drake (Lonette McKee), an attractive woman and law student clerking in the firm, who Monty is instantly smitten with. Monty tries to convince her that he's not another newly rich snob and asks her out, thinking he can hire someone to do her work for her. He is coldly rebuffed by Angela, who has a fiance, Warren (a junior partner at the law firm).
The next day Monty has rented a large penthouse in the Plaza Hotel and a large amount of office space in the same building. He has also hired a couple of high-priced tailors to make custom suits for him, renting the clothes themselves. With the salary that Monty is paying him, Spike fixes himself up. Down in the new office that Monty is renting, he meets Angela's boyfriend Warren (Stephen Collins) and his former girlfriend Margaret, whom Monty has hired to redecorate the office space. Unknown to anyone, Granville and Baxter have slyly charged Warren with spying on Monty to make sure he does not complete the challenge so they can get his millions for themselves.
In the office are hundreds of people looking to either do business with Monty or scam him. One man has an investment offer; a large iceberg will be outfitted with giant engines and be sailed to the Middle East where the ice will be used to supply water to the farmers in the region. Monty is also allowed to gamble 5% of his $30 million, so he hires a bookie to bet on every long shot he can. Monty calls his old Bulls coach and tells him he plans to fix up their old playing field in Hackensack and that he'll arrange a three-inning exhibition game where the Bulls will play the New York Yankees.
Monty continues his spending campaign for a few weeks. One night he's inspired by an idea from Spike to invest in expensive items like stamps. Monty goes to a fine collectibles store and asks to see the most expensive stamp the owner has. The stamp is the infamous Inverted Jenny which Monty buys for $1.25 million. Granville and Baxter read the story in the newspaper, thinking Monty has violated the terms of the will. Baxter finds a postcard in the daily mail that has a photo of the Hackensack Bulls and a bland message from Monty. After a few moments, Baxter realizes that Monty has used the Inverted Jenny stamp to mail the card, thereby devaluing it. Both men are furious and have a private meeting with Warren where they show him the will (the conditions of not revealing the $30 million spending plan in 30 days does not apply to the others of the will reading). They then order him to cause a small error in bookkeeping amounting to $20,000 that will be discovered at the last moment. They promise Warren that he'll be made full partner and that Monty's real inheritance will go to the firm.
Spike and the rest of Monty's friends and associates become very worried about Monty's frivolous spending and try to convince him to curtail it. When Monty finds out that the iceberg investment has turned out to be lucrative, he immediately wants to sell his stock in it. Moments later the bookie he hired storms in and tells him he's won a large amount of money on the bets he placed and that he's banned from any more gambling in New York. Monty decides to donate the money to charity (he's also allowed to donate 5% of the $30 million).
Spike and his financial adviser, Eugene (David Wohl), both rush into Monty's office one day with exciting news. Spike had invested a large amount of Monty's money in a venture and earned Monty's company $10 million. Monty becomes upset, realizing that he's "right back where he started" and orders everyone out of his office. On television, he sees a news commentary about New York's upcoming mayoral election. The reporter says that his news station has chosen not to endorse either candidate. Monty enters the race himself, declaring a ticket called "None of the Above" where he wants to buy the votes of New York. The campaign drains much of the $30 million in a short amount of time. When Monty's two opponents, Heller and Salvino, figure out that Monty's campaign could defeat them both, they both agree to sue him for libel. Monty settles out of court for a huge amount of several million.
Meanwhile, Angela has grown increasingly cold toward Monty, thinking he's foolish for spending the $30 million so rapidly. Earlier, Monty had tried to give her a new Aston-Martin convertible -- a violation of the will that Monty can't tell Angela about. While he talks to her on the street, he's overseen by both Warren and Margaret, who mistakenly assume Monty and Angela are having an affair.
The day of the big Bulls vs Yankees game arrives. At the game, the Bulls start out doing very well against the Yankees, until the Yanks fight back and defeat the Bulls. Roundfeld approaches Monty in the locker room and tells him if he wins the election that the $60,000 annual salary given to him as mayor could be considered an asset in accordance with the will. Monty announces to the crowd that he's withdrawing from the election. He also invites everyone to a final party which he'll pay for with his last $38,000.
At the party, Monty plans to get very drunk. Spike and Melvin take up a small collection to give to Monty so he won't be flat broke. Monty refuses the money. Spike suggests that he and Monty buy a nice sports car and leave New York for a new life. Monty still turns him down. Alienated, Spike walks away from Monty for good.
The next morning (on the 30th day), the landlord for Monty's office and penthouse politely evicts him. The tailors take back all the clothes he rented and he finds his old Chicago Cubs jersey in the closet. Monty leaves the Plaza in his old street clothes and wanders off into Manhattan. That evening, a local newscaster, wondering where Monty has vanished to after declaring that he is now flat broke, announces that the mayoral race will be held again in a run-off election after primary election results showed that Monty's campaign had worked to sink the campaigns of Heller and Salvino, with the voters indeed voted for "none of the above". The run-off election will be held in a few days with neither Heller or Salvino running for office ever again.
At 11:45 pm, Monty reappears at Granville & Baxter that night to finalize the will. Before he enters the meeting with the two, he's stopped by Warren who eagerly gives him the $20,000 he'd withheld. Monty is immediately upset thinking he'd lost and, with the clock about to strike the midnight deadline, agrees to sign whatever paperwork the two men have. While Warren watches, Angela suddenly appears. Warren discloses the conditions of Monty's will and shows her the receipt she'd missed for the money he'd given Monty. Angela is furious and rushes in to stop Monty from turning over the money to the firm, saying that Warren had cheated him. Warren tells Monty and Angela that he'd suspected they'd been having an affair all along and Angela tries to hit him. Monty stops her and punches Warren himself, in which Warren promises to sue him. When offered an out-of-court settlement for the money he'd been given, Warren refuses. Monty suddenly decides to hire Angela on retainer for $20,000. She frantically writes him a receipt by the stroke of midnight and Roundfeld declares Monty's inheritance complete. He also promises to investigate Granville and Baxter for any wrongdoing for revealing to Warren the terms of the will. Monty and Angela leave the office together where they agree to talk about his legal options and of his new inheritance.