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, despite 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. As Arnold goes through detention, Harold continuously comes to Arnold to make sure he doesn't tell, becoming more and more agitated as time goes on. Soon, he demands to know why Arnold didn't admit the truth when its completely unfair for him to be punished for something he didn't do, and he says because he's not a snitch. That night, Harold is having trouble sleeping because of the guilt. On the last day of his punishment, Wartz prepares to mark Arnold's permanent record, when suddenly Harold, Stinky & Sid burst in, unable to handle the guilt any longer, tearfully admitting, that they were the "mooners", Arnold had nothing to do with it, and begging Wartz not to punish him any further.
Wartz, who finally finds out the truth lets Arnold off the hook (for now). The boys say Arnold has guts. They decide to put a big “Kick Me” sign on Robert Simmons’s back the next day and Arnold tells them that it’s not a good idea.