It is school picture time and Cartman takes great delight when Kenny poses for his school photo by wearing his parka backwards and having his ass stick out where his face should be. Butters is in trouble with his parents for looking stupid in his photo. When the photo comes out, Cartman pulls off what he thinks might be the funniest thing he has ever done, he gets the photo put on a milk carton as that of a missing child, with "blonde hair and a brown eye," and full rosy cheeks. The milk carton comes out right away and Cartman is beside himself with laughter. The photo hits home in Wisconsin, when a couple believes that the photo might be of their long lost son.
The couple from Wisconsin comes to South Park to see if they can find their son. The couple suffers from TPS (Tarsonic Polarity Syndrome; "buttocks where their heads should be"). Cartman has been laughing non-stop since the milk carton was published and brings Stan and Kyle over when he is told the couple from Wisconsin wants to meet him to discover information what information he may know about the whereabouts of their missing son. When Cartman meets the TPS afflicted Thompsons he suddenly loses his sense of humor; he finds that he is no longer able to laugh and he later claims to have blown his funny fuse. Meanwhile the Thompsons have been sent to Kenny's house. Kyle and Stan bring the other boys (sans Butters who is grounded for his stupid face) to see the Thompsons. Meanwhile, Cartman seeks counseling from Mr. Mackey to try to restore his sense of humor. The Thompsons find out about the prank. The South Park Milk Company, the largest milk company in the country, offers the power of their computer's database to help them find their missing child. Outside the McKormick house, the living room window fills with people trying to get a look at the Thompsons.
Cartman tries to see a funny movie to restore his sense of humor, with no avail. The milk company computer starts running his search. Cartman listens to some of Jimmy the crippled comic's routine, still to no avail. The computer finds someone in its database that possibly could be the Thompson's missing child.
Butters' is still grounded. Cartman writes his mother what might be construed as a suicide note, when in fact it is just a note telling her why he won't be going to school. Without his sense of humor he just doesn't feel he can make it. The milk company reunites the Thompsons with their long lost son who's also afflicted with TPS, This turns out to none other than Ben Affleck, with that news Cartman's sense of humor finally returns.
Kenny dies crossing the street when he is struck by the motorcycle that was reported as running over other residents of South Park.