Randolph Duke was portrayed by the late Ralph Bellamy, and Mortimer Duke was portrayed by the late Don Ameche.
Randolph and Mortimer Duke founded "Duke & Duke Commodities Brokers" together in their youth and by the time of Trading Places had built the firm to great success, making their names notable throughout the financial world. Sometime during their growth they employed the aspiring Louis Winthorpe III (played by Dan Aykroyd), who was eventually promoted to Managing Director of the company.
One winter, however, the Dukes set up a plan to secretly steal the Orange Crop Reports before they were officially released to the public (allowing them to corner the Frozen Concentrated Orange Juice market and make millions on the stock exchange).
They hired Clarence Beeks (a security officer from the company holding the crop reports) to steal the reports on New Year's Eve (two days before they went public).
During a regular business day, Randolph and Mortimer were at a gentleman's club with Winthorpe signing the payroll checks for employees when they falsely accuse an African American homeless man, Billy Ray Valentine (played by Eddie Murphy), of stealing them as Winthorpe left the club.
Following Valentine's arrest, the Dukes decided to perform an experiment based around the Hereditary vs Environment argument (outlined in Randolph's Science Magazine); the two make a bet that Randolph could take Valentine and with good environment, turn him into a successful company executive, while at the same time taking the successful Winthorpe and proving that placing him in the worst environments would turn him into a criminal.
With the help of Clarence Beeks, the Dukes successfully frame Winthrope for dealing drugs and have him fired, arrested and cause his fiance to leave him while at the same time taking Valentine off the street and settling him into Winthropes house with his former butler, Coleman.
During the course of the experiment Winthrope descends further into squalor whilst Valentine rises as a success. Eventually on Christmas Eve, Winthrope storms Duke & Duke during the Christmas Party, armed and intending to plant drugs in Valentine's office. He has caught in the act by Valentine and the Dukes before leaving frantically waving a gun around.
With this Randolph won the bet and the two brothers decide to settle up in the company restroom and here unknowingly reveal their whole experiment to Valentine (who was secretly smoking marijuana in one of the stalls).
Now aware that he and Winthorpe had been cruelly used, Valentine went to Winthorpe where the two of them plotted their revenge against the Dukes. After figuring out the Dukes plan to steal the crop reports, Winthorpe and Valentine manage to trail and incapacitate Beeks on News Year Eve, before delivering a fake crop report to the Dukes.
Two days later, the Dukes arrive at the stock exchange and using details from the fake report, tell their runner to buy as many Orange Juice contracts as soon as trading started. Meanwhile, Winthorpe and Valentine arrived at the stock exchange armed with the real knowledge.
During trading, the Dukes watch in horror as they buy hundreds of contracts only to find that the information they received (stating that the orange crop was dire) was fake as the true report was delivered live, stating the orange crop hadn't been harmed by the cold winter and therefore causing the price to drop and despite their best efforts to sell their contracts back, are left with a debt of $394 Million Dollars.
With this enormous debt, the Dukes are ruined and lose everything, being forced out onto the streets.
A few years later, Randolph and Mortimer are living homeless in New York when a man (a prince also played by Eddie Murphy) gives them a huge amount of money (claiming it to be "pocket change") resulting in Randolph stating "Mortimer....We're Back!".
Randolph Duke is a very laid-back and unexcitable man who enjoys the rich life. He appears to be keen at mathematics, performing quick calculations over the companies profits. He is also keenly interested in science and enjoys reading and discussing experiments from published journals (much to his brother's chagrin).
Mortimer Duke is more stern and greedy than his brother; he spends most of his time reading financial papers and cares little for the affairs of anything but his company. He is also shown to give very cutting and opprobrious remarks both on the subject of race and class, although this probably changed in "Coming To America".
Both of them are extremely selfish and show psychopathic tendencies, framing Winthorpe and robbing him of everything in his life just because of a bet over merely one dollar, with Randolph even stating they had done it before to other people just for the heck of it.
- 30 years following the creation of Trading Places, the implications of the Duke plan could actually have been carried out in the real stock exchange and was therefore banned under what became known as "The Eddie Murphy Rule" under Section 136 of the Wall Street Transparency and Accountability Act.