It's school picture day and Kenny McCormick has decided to pull a prank. He went into his coat upside-down so that his butt would show instead of his face. Once the picture is taken, Eric Cartman, without telling his friends, takes the picture to the South Park milk company, telling them that he's a lost child. They print out the picture on all cartons of milk whereupon a Wisconsin family, who actually have butts for faces, see the carton and come to South Park to find their missing son. Meanwhile, Butters is grounded for making a "stupid face" in his school photo, despite his protests that he wasn't.
The episode begins with the South Park children having their class pictures taken, and we see Stan and Kyle's entire hair for the first time. Cartman smiles and poses, and Butters is seen combing his hair, although one flick of hair does stick up on his actual picture. During the course of this, Kenny wears his parka upside down and stands on his head with his feet through the sleeves so that his "ass shows through his hood."
Ms. Choksondik tells the class that one of the boys spoiled his picture, and therefore he won't be getting it back. After Kenny says "Oh, come on!", it actually turns out to be Butters, despite the fact that Butters didn't make any silly faces; it's how he looks. Ms. Choksondik and Mr. and Mrs. Stotch inevitably remain convinced that he did make silly faces, though, and he gets grounded for it. This leads Butters to reclusion and self-loathing.
Cartman is pleased with the trick he made Kenny do, and he wants to extend it even further. Then he takes the photograph to a missing children's center, and he gives them a description of the child, consisting of a "winking brown eye", "blond hair", and "rosy cheeks." Rather than realize it's a trick, the receptionist at the counter falls for it, and the photo gets placed on a milk carton.
Astoundingly enough, the photo draws a response from a couple, Martha and Stephen Thompson, who lost their son several years earlier. They suffer from a fictitious congenital condition known as "torsonic polarity syndrome" or "TPS" which has them born with their faces replaced with human buttocks. With the syndrome, however, they still retain all normal functionality of their faces (including noses, eyes, etc.).
Cartman is stunned when the Thompson's show up at his place, asking if he had seen their son anywhere; so much so that he finds he can't laugh at their condition. He becomes convinced that he has now seen something so funny, it blew his "funny fuse", and now he'll never be able to laugh at anything again.
Everyone in town is fascinated by the "ass people", although they try to cover up the fact that they noticed their condition - despite the fact that Martha and Stephen seem proud of their condition, stating "over eleven people worldwide have this condition!". Kyle and Stan take several kids along with them, and soon Mrs. Broflovski, Mr. Garrison, Officer Barbrady, and Chef join in, looking in at the McCormick's window while Kenny apologizes to the Thompsons. They're baffled by the sight.
Cartman, meanwhile, is having no luck restoring his funny fuse; not even Jimmy's jokes can help.
The search for the Thompson's missing son goes on. Then something amazing finally turns up, after days of tension and suspense. The final scene has the lady from the milk company state that the search results revealed that the Thompson's son was Ben Affleck. He and his parents are reunited, and everyone finally notices the resemblance. Cartman makes a comment about Ben Affleck, and how everybody ought to call him "Ben Assfleck" from now on. This triggers his fuse, and he starts laughing again. Stan and Kyle decide that he couldn't laugh before because he actually felt guilty for making the Thompsons believe their son had been found – which Cartman vehemently denies. Even so, Kyle finds it remarkable that Cartman actually felt bad for someone.