When the town council decides to get rid of an old Civil War cannon, the Mayor calls upon Andy and Barney to sell it. After exhausting every possible buyer he can think of (including an indignant Barney), Andy feels his fortunes have turned around when an antique dealer just happens to stop by Walker's Drug Store. Forgetting the talk he just had with Opie about "the golden rule" and how "honesty is the best policy", Sheriff Taylor then commences a bit of what he calls "horse trading" with the dealer, making outrageous claims about the cannon - everything from it having fired the first shot at Ft. Sumter to being the personal canon of Teddy Roosevelt during the Battle of San Juan Hill.
Not only do Andy's underhanded tactics upset Barney, but they have a profound impact on Opie - who sees an opportunity to unload some worthless merchandise on his own. After Andy sells the canon for $175, neither Ellie nor Barney will speak to him. Upset that they refuse to see how he made good money for the town and got rid of an eyesore at the same time, he storms out of the drugstore only to run into Opie - who is sporting a brand new pair of roller skates and enough extra money for an ice cream cone. Andy questions Opie only to discover that Opie has been doing some "horse trading" of his own.
After a short and pathetic attempt at justifying his trading tactics while condemning his son's, Andy comes to see the error of his ways. He then refuses to sell the canon even though the antique dealer still wants it. In the end, Andy agrees to sell the cannon - but only for $20. However, like that famous bad penny, the cannon's not gone after all - the antique dealer sells it, using Andy's lies, to a man who intends to give it to Mayberry!