In the heart of a bustling Philadelphia, the intriguing Duke brothers, Randolph and Mortimer, engage in a high-stakes wager that pits the age-old debate of nature versus nurture against each other. In a bold and audacious experiment, they decide to swap the lives of two men from completely different backgrounds: the sophisticated and well-mannered Louis Winthorpe III, played by Dan Aykroyd, and the cunning street-smart hustler Billy Ray Valentine, brought to life by Eddie Murphy. Their scheme kicks off when an unexpected encounter between Winthorpe and Valentine leads to Valentine’s arrest, with Winthorpe mistakenly believing he is a victim of robbery.
As fate would have it, the Dukes recruit Clarence Beeks, a man on their payroll, to execute a diabolical plan aimed at tarnishing Winthorpe's reputation. Through a series of manipulative tactics, Beeks frames Winthorpe as a drug dealer and criminal, leading the Dukes to cut off his access to funds, terminate his position at Duke & Duke Commodity Brokers, and abandon him to public disgrace. Meanwhile, Valentine finds an unlikely ally in Ophelia, a prostitute who supports him in his quest for justice and seeks monetary reward for her assistance. Pleased with the progress of their experiment, the Dukes decide to bail Valentine out and elevate him to Winthorpe's former role within the firm.
In the days that follow, Valentine flourishes as an emerging business talent, deftly applying his street smarts and evolving his persona to fit the corporate world. However, during a Christmas celebration at the firm, Winthorpe devises a scheme to sabotage Valentine by planting drugs in his desk and making a dramatic escape. This incident sparks a conversation between the Dukes as they discuss their little wager — only to plan on sending Valentine back to the streets and keeping Winthorpe permanently benched. Unbeknownst to them, Valentine overhears this sinister plot and rushes to save his former adversary, who has just attempted suicide.
With the timely intervention of Ophelia, Valentine, and the steadfast butler Coleman, Winthorpe begins to mend his wounds and learns of the Dukes' manipulative experiment. It is a news segment regarding the USDA's predictions on orange crops that ignites the fire within both men to conspire against their manipulative captors. They realize that Beeks holds the key to the Dukes' plans, as he carries the confidential report that can help corner the orange juice market.
As New Year's Eve reaches its crescendo, Beeks finds himself unexpectedly ensnared in chaos when he is duped by four clever board members posing as traders, seeking to replace a genuine report with a deceptive one. The plot thickens when Beeks is knocked out by a gorilla, setting up an absurd twist that allows the impostors to proceed with their scheme. In the meantime, Winthorpe and Valentine hatch a plan to outsmart the Dukes by investing heavily in frozen concentrated orange juice futures. Their tactical move draws in other traders, creating a whirlwind of market activity.
After a series of intense trading maneuvers, the market takes a nosedive when the actual crop report reveals a normal harvest. Seizing the opportunity, Winthorpe and Valentine recover their short-sold contracts at a lower price, excluding the Dukes, thus fulfilling their obligations while raking in an immense profit.
Once the dust settles from this financial rollercoaster, the duo confronts the Dukes, revealing their gambit: to amass wealth while orchestrating the Dukes' downfall. Faced with a margin call of an astonishing $394 million, the Duke brothers find themselves destitute, their world crumbling around them. As Randolph clutches his chest in despair and Mortimer rails against the futility of their situation, Winthorpe, Valentine, Ophelia, and Coleman toast their newfound prosperity while embarking on a luxurious getaway, leaving behind Beeks and his gorilla friend bound for adventures in Africa.
