Although David is really more mischievous than malicious, his idea of amusing himself is playing jokes on others. On the morning of the all-school hobby day, he scoffs at the doll clothes his younger sister Sandy has made to show to her class, and she gets back at him with a prank that spoils his breakfast. Their mother scolds her, cautioning that "jokes are all right if they don't hurt anybody." David rushes off to school without breakfast and along the way teases a girl by grabbing a package from her and running off with it. During the course of the day he tricks a candy store clerk, snatches away a classmate's glasses, and puts a sticky sign on another student's desk seat. There is a turn of events when David gets up before the class to discuss his own hobby and show a model airplane. So far David hasn't learned that what seems funny to him isn't very funny to anyone else. Suddenly, as he looks into his package, he imagines vividly what might have happened if each of his jokes had turned out differently. When his daydream is over, David discovers that although he's been the joker, someone else has had the last laugh.