The film revolves around Randolph and Mortimer Duke, two incredibly rich speculators who operate on the derivatives market (when this film was made (1983) the futures market was still in its infancy, and you can learn a lot about the business from the film!)
They have a debate - not a new debate, but a thorny one nonetheless : is ability inherent in the way you are brought up (i.e. nuture), or is it inherent in your breeding (i.e. nature). They decide on a little experiment.... enter our two heroes.
Eddie Murphy plays a down and out, living on the streets, doing what he can to survive. Dan Ackroyd plays a futures broker, rich, successful, a huge house and butler, and a beautiful wife. Randolph and Mortimer decide to swap them (with the help of a unsavoury private eye named Clarence Beaks played superbly by Paul Gleason). Eddie becomes the high flying futures broker, and Dan becomes the down and out. As you can imagine, hilarity ensues!
There are many other reasons to watch this film, including superb performances from the likes of Don Ameche and the sadly departed Denholm Elliot, and of course the gorgeous Jamie Lee Curtis ensuring interest rises by revealing her assets (sorry - couldn't resist the banking pun).
PS : For those wonder how the Dukes turn out after the film, don't worry too much; watch Coming To America to see an inspired cameo!