Titled Gnomes, we find one of the main character's underpants (seriously) being stolen by small gnomes who feel compelled to do so. When finally confronted, they explain that it is central to their three part plan:

The entire episode was contained within the usual countercultural paradigm that the show so often seeks out; in this case it was the WalMart-phobia of the 1990's where especially small towns would rise up against the invasion of the big box retailers (in this case it was a parody of Starbucks) who everyone knew put mom and pop out of business. Depicted often as heartless corporations, the South Park version is a defense of where capitalism inevitably goes; just as family farms were lost in the industrial progress of the 19th century, so were the mom and pop retail shops of the late 20th.

The gnomes were business geniuses who just couldn't explain why (as shown above), representing the hidden hand of capitalism in which individual actors often aren't entirely sure exactly how they are successful – they just are.