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 four people who stayed after school: Gumball (who had detention), Penny (who was doing "cheerleader stuff"), Rocky (who was cleaning), and Darwin (who was unable to remember). 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 did not 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 does not 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 could not 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, Carmen reminds Darwin that he could not have done it, because she 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, Gumball can be heard running away and Miss Simian realizes he is not 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, obstructing justice and 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 flinching at the latter event.
The sequence where Darwin runs around with toilet paper on his shoe is a reference to the comedic sketches on The Benny Hill Show. Likewise, the accompanying music parodies "Yakety Sax," a song used during the said show.
The crushing sound heard when Principal Brown and Miss Simian are hit by a vehicle is used in the Grand Theft Auto series of video games, when pedestrians are run over.
The ending scene, in which Miss Simian is run over by an ambulance is a reference to the Japanese manga Jojo’s Bizarre Adventure: Diamond is Unbreakable, in which the main villain, Yoshikage Kira, is defeated by getting run over by an ambulance.
Darwin and Penny did not appear to chase Gumball, but they are seen with the group at the end of the chase.
When Bobert is first seen on the stairs when everyone enters the classroom he appears to be fully active, but he later states that he blew a fuse and was turned off.
When Gumball and his friends examine Principal Brown, Molly appears out of nowhere.
In the same scene, her mouth was pixelated for a split second.
Sussie disappears from the group of students after they chase after Gumball (possibly due to the animators not knowing how to do Sussie's running animation.)
During the first display of Bobert's memory, no one can be seen opening the door, but during the second time, it shows the culprit quite clearly.
When Miss Simian is yelling at the children at the beginning of the episode, her face is a darker shade of grey than usual.
In "The Robot," it is stated that Bobert cannot feel emotions, but in this episode, he finds something humorous and laughs at it.
It does not make sense that Rocky was persuaded to chase Gumball by Miss Simian's promise of no homework, since he is a janitor and not a student.
When Banana Joe crashes into Anton's face, Anton's face is covered with squashed banana, but when Anton, Idaho and Tobias are chasing Gumball on the ladder, Anton's face is spotless.
In the library, Tina is hit by a bookcase, but afterward, in the corridor, she is seen chasing Gumball again. However, it is possible that she got out.
In the scene where Rocky tackles Brown, his pelvis is invisible.
During the scene when Brown walks by Rocky, footsteps can be heard. Brown does not have any feet, therefore he cannot wear shoes.
Assuming the yearbook is arranged alphabetically, Darwin and Anais are out of order, with Darwin coming before Anais.
In Bobert's replay when Principal Brown is falling down the second flight of stairs, Gumball's head is invisible. After he crashes into Gumball's locker, the mistake has corrected itself.
When Bobert narrates Gumball greeting Penny in the evidence replay, he states that "[Gumball looked] lovestruck, as usual," even though Bobert does not know what love is, as stated in "The Love."
When Principal Brown asks where Miss Simian went, he has a mouthful of sharp teeth.
At the start of the episode, the stairwell can be seen from the classroom. During the chase scene, a wall replaces the stairwell.
Čeština (Czech): Záhada (The Mystery)
Español (Spanish): El Misterio (The Mystery)
Français (French): Le mystère (The Mystery)
Italiano (Italian): Il mistero (The Mystery)
Magyar (Hungarian): A rejtély (The Mystery)
Português (Portuguese): O Mistério (The Mystery)
Polski (Polish): Tajemnica (The Mystery)
Українська (Ukrainian): Загадкова пригода з містером Брауном (Mysterious Adventure with Mr. Brown)
Leslie: Gumball, have you seen your yearbook photo?
[Leslie holds up a yearbook and points at Gumball's photo. He has a derpy pose and face on the photo]
Gumball: No interruptions, please. [In deep voice] We all know how handsome...
[Back to recreation]
Nigel Brown: ...Gumball is, but he's still in detention!
Penny: Then you leave me no choice!
Gumball: It was then that you pompom'd him unconscious-
[Penny hits Principal Brown with her pompom]
Gumball: Shaved off his hair to punish him, covered him in paint to humiliate him, wrapped him in toilet paper to keep him quiet, and stuffed him in my locker as proof of your deranged obsession for me.
[End of recreation]
Gumball: It was you!
Gumball: But I forgive you, because I am madly in love with you too.
Penny: Yeah, well, that's not quite what happened.
[Flashback to yesterday at the school stairwell, Penny hangs her finished painting]
Penny: I had finally finished my painting of the cheerleaders, and I went to hang it in the stairwell. I guess it was still wet, and I must have got paint on my pompoms.
Gumball: And did anyone see you supposedly hang this alleged painting in this "so-called" stairwell?
Penny: Yeah. You did.
[Flashback begins. Penny hangs her finished painting, as Gumball comes around the stairwell]
Gumball: Hey, Penny! I love you! Your painting! Your painting. I love your painting.
[Gumball facepalms, end of flashback]
Gumball: [laughs nervously] Oh, yeah.
[The spotlight switches off]
Alan: So if Penny didn't do it, who did?
Gumball: Hmmm. It must have been...you!
[Gumball points to Rocky, but the spotlight does not switch on]
Gumball: I said it was him!
[The spotlight shines at Gumball but slowly moves to Rocky. Rocky and the students gasp when the spotlight shines on Rocky]
Rocky: Why me?!
[Another recreation begins]
Gumball: Yesterday, you were wandering around when suddenly, at the age of twenty-seven, you've realized your youth was behind you.
Gumball: Then all your hair fell out.
[Rocky's hair falls out]
Gumball: When you saw Principal Brown's lustrous hairy coat, you were overcome with jealousy. And acting on impulse, you attacked him. You frantically plucked out all of his hair, dyed it the right color, and used it to cover your balding shame. Then, planning to mislead any investigation, you painted Principal Brown, wrapped him in toilet paper, and stuffed him into my locker, the perfect crime, save for one thing!
[End of recreation]
Gumball: When you stuck the new hair on your body, there wasn't quite enough, which explains this bald spot on your hand!
[The students gasp]
Rocky: Dude, you've got it all wrong! I got this bald spot when I was cleaning yesterday. You see, I spilled some really strong chemicals on my hand, and it burnt off the hair. See? [Rocky spills the chemicals onto his hands] Aaaaaahhh!
Gumball: If that's the case, how come I can just pull your hair right off?
[Gumball pulls Rocky's hair]
Rocky: Agh! Oh! Stop! Ow! No! Agh!
Gumball: Oh, okay then, I guess it wasn't you.
[The spotlight switches off]
Darwin: Who could it be? All this suspense is making me sweaty.
Gumball: Why don't you just wipe your brow with that toilet paper suspiciously stuck to the bottom of your shoe? [Gasps] Clue! [Gumball swells up tears] I can't believe I'm gonna have to say this, buddy, but, it was you!
[The students and Darwin gasp and the spotlight shines on Darwin]
Darwin: Yes! Yes! I did it! I'm a monster! A monster! Just lock me up. I'm a menace to society. [Darwin falls on his knees, sobbing. He then plops on the floor] Take me away!
Gumball: I rest my case.
Carmen: But Darwin couldn't have done it!
[The spotlight switches off]
Leslie: We all saw you on the bus when we went home.
Darwin: Then how'd you explain the toilet paper?
[The spotlight shines on Darwin again]
Tobias: You've had it on your feet all day, dude, didn't you notice?
[Flashback to where Darwin is walking with toilet paper on his shoe, leaving a very long trail as he goes into different rooms. He goes down the hallway, up the stairs, down the stairs, back down the hallway, then onto the school bus, as the shutting door cuts it off]
[The spotlight switches off]
Darwin: Don't remember any toilet paper.
Gumball: So it wasn't Penny, it wasn't Rocky, and it wasn't Darwin. Which leaves us with only one suspect. The only person who knew the combination to the locker Principal Brown was found in. You!
[The students gasp as Gumball points at himself and the spotlight goes on him]
Gumball: Oh, heh heh.
[The students go "Hm!"]
Lucy Simian: That's all the proof I need. You're coming with me, Watterson!
Gumball: Wait! You're forgetting one bit of crucial evidence!
Lucy Simian: And what's that?!
Gumball: I'll just...go and get it.
[Gumball runs away]
Lucy Simian: He's not coming back, is he?
Lucy Simian: No homework for the one who catches him!
[The class runs off]
[Gumball runs to the library. Molly, Leslie, Tina, Rocky, Tobias, Idaho, Carmen and Anton are blocking him, then Molly kicks Banana Joe but Gumball jumps over and Banana Joe lands in Anton's face becoming a Banana toast spread. Gumball then jumps over Molly and Leslie before jumping on a ladder]
Gumball: Ahahaha! Ow!
Tobias: Quick! Give me something else to throw!
[Tobias and Anton look at Idaho]
Idaho: What? [throws Idaho] Aah!
Gumball: Stop throwing stuff at me, I'm innocent!
[Gumball, Tobias, and Anton fly to bookshelves, causing Carrie, Juke, and Teri to look up from their books then Gumball jumps from the falling bookshelves, Tina tries to bite Gumball and misses, causing the bookshelves to fall on her]
[Gumball is back in the hallway, running from Tina, Tobias, Anton, and Leslie]
Gumball: Leave me alone! I'm innocent! I would never hurt Principal Brown!
[Scene cuts to Principal Brown on a metal bed in the nurse's office.]
Nigel Brown: I think I'm feeling a little better.
[Gumball runs into his bed, causing him to crash through the window]
Nigel Brown: Aaaaaah! [Lands on the ground]
[The students gather around the broken window, as Gumball looks at them nervously and sneaks away]
Nigel Brown: This is extraordinarily painful. It couldn't possibly get any worse!
[A school bus reverses and runs over him, offscreen. A crunch sound is heard]
[Gumball runs into Bobert]
Gumball: Bobert? Ah! Don't turn me in! Oh, you've been switched off.
[Gumball turns Bobert on]
Gumball: Ah! Don't turn me in!
Gumball: Principal Brown was attacked, and everyone thinks it was me!
Bobert: But it wasn't you.
Gumball: Exactly! Wait, how come you believe me?
Bobert: I saw...everything.
Lucy Simian: Gumball! Get him!
[Everyone runs toward him]
Gumball: Sto-o-o-o-o-op! New evidence has come to light! Tell them what happened, dude!
Bobert: I can do better than that. I can play you my memory. [Projects a hologram to show the footage as he describes it] First, Penny hung her hideous painting on the wall.
Bobert: Then, Gumball turned up, looking love struck as usual.
Bobert: Next, Darwin walked down the stairs, trailing toilet paper like a nincompoop.
Darwin: What's a nincompoop?
[Gumball whispers "Somebody like you" in Darwin's ear]
Bobert: Then, Rocky dropped a bottle of chemicals on the floor, looking quite old.
Bobert: And finally came Principal Brown, whose office door suddenly flew open, leaving him disoriented and dizzy. He fell down the stairs, rolling himself in Darwin's toilet paper, smashed into Penny's artwork, covering his face in paint, then fell back into Rocky's chemicals, burning off his hair, then down more stairs, wrapping himself into even more toilet paper, and finally into Gumball's locker.
Penny: But why didn't you say something before?
Bobert: Well, it was so funny, I blew a fuse. I was like, "Ha. Ha. Ha. Ha." [Bobert blows a fuse]
Gumball: He's done it again.
[Gumball turns Bobert back on]
Penny: So, the true culprit is the person who threw open Principal Brown's door!
Gumball: Exactly. And I know just how to find out. Whoever opened the door, put your hand up. [No one raises their hand] Oh, come on!
Bobert: Why don't I just replay the tape?
[Replays the footage quickly]
Gumball: Stop there! [Stops the video on the part of Principal Brown walking down the hallway] Now forward slowly. [Footage slowly moves] Freeze! [Video pauses on the door swinging open, hitting Principal Brown] There. Zoom in fifty percent. [Does as told] Zoom in one-hundred percent. [Does as told] Zoom in another one-hundred percent. [Does as told] Enhance video.
[It was Miss Simian who opened the door and the students gasp]
Gumball: It was you all along! We're going to tell Principal Brown!
Lucy Simian: Tell him what?
Gumball: What we just saw on Bobert's memory.
Lucy Simian: What memory?
[Miss Simian erases Bobert's memory]
Bobert: Erase memoryyyyyyy. [Slowly] Who...am I?
Darwin: That's not fair!
Gumball: How do you sleep at night, Miss Simian?
Lucy Simian: Like a baby. [Laughs evilly]
[Principal Brown opens his door suddenly, which knocks Miss Simian offscreen]
Lucy Simian: Aaaaaah!
Nigel Brown: Has anyone seen Miss Simian?
[Miss Simian crashes through a window]
Lucy Simian: Aaaaaah! [Lands on the ground]
[The students gather around the broken window.]
Darwin: Don't worry, Miss Simian! We just called an ambulance!
[An ambulance reverses and runs over Miss Simian, offscreen. A crunch sound is heard]