Harold, Sid and Stinky moon Principal Wartz, but Arnold is blamed, even though he was just an innocent bystander.
Arnold is walking to class when he sees Harold, Sid, and Stinky at the corner. They plan to moon Principal Wartz. They ask Arnold if he wants to join in, but he thinks it's not a good idea. The three tell him he just doesn't have any guts.
Wartz leaves his office and starts walking toward them. Harold, Stinky and Sid wear masks to disguise themselves. Wartz walks by and they unbuckle, unzip, and pull their pants down, fully exposing their butts. Wartz puts on his glasses and is left shocked at the sight. The boys run away laughing, while Wartz tries to stop them. However, they get away and Wartz takes Arnold to his office, unaware that Harold, Stinky and Sid are the culprits.
Arnold says he didn't do anything, but Wartz says he did see who did it, which Arnold agrees. Wartz asks Arnold who did it, but Arnold won't tell because he's not a snitch. Wartz assigns Arnold four weeks detention for failure to cooperate. Arnold leaves while Wartz makes a phone call. Sid, Harold, and Stinky arrive and ask Arnold if he snitched. Arnold says he didn't, which the boys are happy about. Arnold tells them Wartz gave him four weeks detention, but Stinky says he didn't even do anything.
Sid and Stinky, feeling guilty, think about telling Wartz the truth, but Harold, determined to stay out of trouble, refuses, saying Arnold getting caught is his fault, depite him not participating in the prank. Throughout the next four weeks, Arnold goes through many punishments, but still refuses to tell who did it, even when Wartz threatens to mark his permanent record, still unaware that Harold, Stinky and Sid are the culprits. Harold asks why Arnold didn't admit, and he says because he's not a snitch. That night Harold is sleeping but is having guilt. The next day, Wartz prepares to mark Arnold's permanent record, when suddenly Harold, Sid, & Stinky burst in, admitting in tears, that they were the "mooners" and Arnold had nothing to do with it.
Wartz, who finally discovers the truth lets Arnold off the hook (for now). The boys say Arnold has guts. They plan on pulling another prank and try talking Arnold into it.