Two wealthy business men switch the places of a street bum and a socialite as part of a bet about heredity vs. environment.
Excellent modern day updating of Mark Twain's The Prince & the Pauper, with a dash of his Million Pound Bank Note and Mozart's The Marriage of Figaro tossed in (the music over the main titles is no accident).
Probably what makes this movie so great, besides the performances, is how satisying the revenge is. The Dukes are thoroughly evil old money one-percenters, and to see them get their comeuppance is a large part of why this movie is still popular three decades later. It's also a great Christmas movie (unlike, say, Die Hard).