The episode starts with Gumball unlocking his locker while talking to Darwin about cheese and cake. When Gumball opens his locker, something wrapped in toilet paper falls out. A crowd gathers, commenting on the gross smell and look of it. As Tobias pokes it with a stick, Miss Simian appears, angrily asking what the fuss is all about. She, too, is grossed out by the object. After ripping off part of the toilet paper wrapping, it is revealed that Principal Brown was the one wrapped up, and that he was also shaved and painted green. Miss Simian cradles the principal, then demands to know who caused this to happen. No one owns up, so she orders all of them to get into her classroom, including Rocky , who was cleaning. Miss Simian threatens to put everyone in detention if no one admits to doing it by the time she gets back.
After she leaves, Gumball asks everyone for their opinion on the subject. Sussie saw Principal Brown after school, which means that whatever happened to him occurred after school. There were only three people who stayed after school - Gumball, who had detention, Penny , who was doing cheerleader stuff, and Rocky , who was cleaning. Darwin was unable to remember, so he was a suspect as well. Banana Joe convinces everyone that it was Gumball , because the Principal was found in his locker, which causes everyone to glare at Gumball. He denies doing it, but promises to find out who did. He first tries to find the culprit by telling whoever did it to raise their hand. Obviously, no one raises their hand. He then tries to get everyone who didn't do it to raise their hand. Everyone raises their hand except for Alan , which causes everyone to look at him. Gumball accuses him, but Alan , annoyed, bumps Gumball and says that he doesn't have any hands. Penny tells Gumball that he should look for clues.
The first clue Gumball finds is, coincidentally, the green paint splats on Penny 's pom-poms. He invents a humorous story to describe why Penny would do this to Principal Brown , but Penny reminds him that he saw her after school, which means she couldn't have done it.
The next clue was a patch of missing hair on Rocky Robinson 's hand. He again invents a humorous motive behind the assault on Principal Brown , but this story is also proven false.
The third clue was a sheet of toilet paper stuck to Darwin 's shoe. Gumball has trouble calling out his best friend, but he painfully brings himself to do it. Darwin immediately admits to it, calling himself a monster. As Gumball closes the case, everyone reminds Darwin that he couldn't have done it, because they saw him on the bus after school. The toilet paper was stuck to his shoe all day.
Gumball was the only other person who stayed after school, which caused him to accuse himself, causing his classmates to turn on him. Miss Simian arrives just as he does this, which is enough proof for her to give him detention, but Gumball was able to buy himself some time by saying he was going to get a crucial piece of evidence. After a while, we hear Gumball running away and Miss Simian realizing he wasn't coming back. She offers a reward of no homework for whoever catches him, which leads to a big chase through the school. Gumball escapes and bumps into Bobert , who was deactivated. Gumball turns him back on, and he says he knows who caused Principal Brown 's injuries, which is when Gumball gets surrounded by his classmates, all eager to claim the no homework reward. Gumball begs Bobert to tell everyone who did it, and Bobert shows everyone a video, revealing exactly what happened. Principal Brown was walking down the hallway when suddenly, his office door flew open, leaving him dizzy. He then fell down the stairs, wrapping himself with a toilet paper trail that Darwin left behind, and slammed into Penny 's wet painting, which covered him in green paint. Next, he fell face first into a puddle of hair-loss chemicals that Rocky spilled, which caused him to jump up in pain and fall down more stairs, right into Gumball 's locker. Gumball just shrugged and walked away. Everyone agreed that whoever opened the office door was the one responsible, so Gumball replayed Bobert 's memory, zooming in on the person who opened the door. It is revealed to be Miss Simian . She erases Bobert 's memory to stay innocent, causing Gumball and the rest to turn on her. As she walks away and taunts the children, Principal Brown opens his office door and pops out. The door hits Miss Simian , sending her flying out the window. The kids peek out the window to see Miss Simian getting run over by the ambulance that was called for Principal Brown . The episode ends with the kids flincing at the latter event.