On Monday, Richard and Anais are having breakfast, when a grumpy Gumball and Darwin stomp in, splatter breakfast all over their faces, and then leave the table angrily. Anais asks Richard to hug her brothers, to which he obliges. He approaches them, and offers them a "Wonder Hug." The boys are skeptical, but he hugs them anyway, causing a large rainbow mushroom explosion in the Wattersons' house.
Less than two hours later, at Elmore Junior High, Miss Simian basks in everyone's misery via a closed-circuit television monitor, while playing a sad song on a cassette-player. She notices Gumball and Darwin being unusually happy, and decides to investigate. In the classroom, Gumball and Darwin are auto-tune singing in the classroom, and Miss Simian wonders what is wrong with them. Gumball places Darwin's arm on his face, thinking it is a nose, making Gumball laugh hysterically and coughing up rainbow liquid, much to Miss Simian's horror. She rushes to Principal Brown, who is caught taping himself, and she warns that there is an "outbreak of joy." He does not believe her, but Miss Simian is determined to prove its existence, taking Principal Brown's camcorder with her.
At the school clinic, Miss Simian begins her documentary recording of the purported disease, much to the school nurse's annoyance. Hours pass, and the still joyous Gumball and Darwin only get worse, and, eventually, Miss Simian isolates them in quarantine. Darwin hallucinates and he and Gumball try to kiss Miss Simian, but get drenched with cough syrup. While the nurse goes to get Principal Brown, Miss Simian observes the boys passing out from the Joy Virus, solidifying her evidence of its outbreak. She excitedly shows Principal Brown and the school nurse, but Principal Brown is concerned by this and opens the quarantine. This allows Gumball and Darwin to spread the infection out. Principal Brown and the school nurse are immediately attacked and infected.
Miss Simian flees to her office, and watches in horror, as the infected victims spread the Joy Virus via hugging everyone else. She wrecks her office out of despair when she accidentally plays the same song on the cassette-player from earlier, which subsides Principal Brown's disease. She realizes the cure, and cautiously treads the halls while avoiding the infected. She pretends to be infected herself while sneaking past the other infected people, but blows her cover when she confronts Banana Joe, who has once again violated the school dress code. The infected chase her all over school, but she successfully reaches Principal Brown's office, locking herself in. She desperately waits for the cassette-player to rewind, but it is too late, as Gumball and Darwin, who are already in the office, attack her.
Miss Simian records her final moments on Principal Brown's camcorder as she slowly gets "hugged" by the Joy Virus, saying that "this story has come to a happy ending," and laughs hysterically, while rainbow-colored liquid spews out of her mouth. After her tape ends, a brief pre-recorded clip of Principal Brown fooling around with a broom plays, ending the episode.
The music heard when Gumball's face turns around is similar to the theme of Disney's The Haunted Mansion.
When Rocky is infected by the nurse after putting on body spray, he says "Oh, it really works!". This is a reference to Axe Body Spray commercials showing women being attracted to men who use it.
The rainbow liquid splattering on the camera while Principal Brown was being infected was a reference to a cliché from many zombie and horror movies, where blood splatters on the screen.
The home-camera scenes are a possible reference to the film Cloverfield.
When Gumball puts Darwin's arm on his face, it resembles Pinocchio's nose.
When Darwin was hallucinating, thinking Miss Simian is a stuffed cuddle monkey and the School Nurse as a candy cane, the stuffed monkey is a Sock Monkey.
The virus is loosely inspired by the creatures from the 1977 horror movie Rabid.
The episode uses the 24-hour clock, which is used in the United Kingdom where the show is produced, instead of the 12-hour clock, which is used in the United States, when noting time. However, the episode incorrectly attaches superfluous "a.m." and "p.m." to 24-hour clock time notations.
According to the time that appears on the screen, the infection started spreading to the entire school after 16:36pm. However, when Miss Simian looks to the security cameras, the time shows 10:17.
When Tina attempts to hug Miss Simian, her eyes are normal; when the mob tries to break into Principal Brown's office, Tina's eyes are missing and have two glowing red dots.
When Miss Simian slams the door on the infected, Rocky's eyes are normal.
In some scenes, Darwin has two eyelashes, but when he was infected, he has three eyelashes, like in season one.
Miss Simian leaves the camera in the nurse's office prior to the infection spreading, but it appears at the end of the episode.
When Gumball and Darwin are laying in the beds, Gumball's eyebrows disappear. This also happens with Darwin when he and Gumball freak out.
When Miss Simian is seen getting surrounded, in one scene Penny is missing her arm bands.
[In front of the Watterson household, a dripping bright red timestamp says Monday, 07:53 AM. Richard and Anais are at the dining room table. Anais eats breakfast while Richard plays a handheld game. Gumball and Darwin stomp to the table, grumpy. They splash their food and milk all over their faces, then dab with napkins and stomp off]
Anais: I think someone needs a hug. [Richard hugs her with his left arm while still playing the game] Not me, them!
[Gumball and Darwin repeatedly stomp and slam the front door. Richard walks up to them]
Richard: What's up?
Gumball: [Grumpily] It's Monday!
Richard: I see. Do you want a hug?
Gumball: There's no point, everything on Monday's crummy!
Richard: What about a wonder hug?
Darwin: [Grumpily] What's that?
Richard: It's filled with all the love and happiness in the world!
Gumball: That sounds crummy, too!
Richard: [Inhales deeply and hugs them] WONDER HUUUUG!
[As Richard hugs them, a rainbow mushroom cloud explodes out of the Watterson household and sparkles are seen everywhere]
[A timestamp says 09:01 AM. Gumball and Darwin are walking down the school hallway]
Darwin: [Smiling] It was a good hug, but I still feel grumpy.
Gumball: Then why are you smiling?
Darwin: I don't know, why are you heel-clicking?
[Gumball is indeed heel-clicking]
Gumball: Yeah. I don't know.
[Miss Simian and Bobert approach each other]
Lucy Simian: You better run, you're late for class! [Bobert begins to run] No running in the halls!
Bobert: Command unachievable, assessing personal level of utilities: zero percent, system obsolete, engaging self-destruct.
[He blows a fuse. Banana Joe is strutting down the hallway to a hip-hop beat with his peel down]
Lucy Simian: Dress code.
[She fixes Banana Joe with a little bit of his insides stuck in a gap in the peel. Joe is seen with a live-action painful look on his face. Miss Simian walks past him and spots a drawing of some children happily playing in the sea on one of the bulletin boards. She draws a shark fin on the drawing, making the children turn and look at it unhappily. In her office, Miss Simian scans through CCTV cameras, with various people exhibiting depressed behavior]
Lucy Simian: [Rocky cries while mopping] Good. [A sad Teri opens a photocopier lid and drops her face onto the scanner, creating duplicates] Hm, excellent. [The Eggheads miserably bang their heads together, cracking each other's shells] Perfect. [Inhales] I love the smell of despair on a Monday morning. But there's something missing. [Giggles] I know. [Presses a button on a cassette player, which plays Beethoven's "Moonlight Sonata"] Yeesss... [Drinks from her coffee mug, which has a sad face on the bottom of it] Hmm?
[She looks at the screen at a camera showing Gumball and Darwin walking to class, looking quite happy. She spits out her coffee through her nose]
[A timestamp says 10:28 AM. In class, Gumball has a big smile on his face]
Darwin: [Also smiling somewhat] Dude, stop smiling, I can feel her eyes burning the back of my head!
[He turns around. Miss Simian is staring through a hole that has been burned into Teri's face. Sarah's head has melted and is boiling]
Gumball: It's not my fault! [Auto-Tuned singing] My cheeks are begging me to stop! But my heart, my heart says keep smiling, homie, yeah!
Lucy Simian: [Storms over to Gumball] HOW dare you sing in my class?
Darwin: I'm sorry, but since this morning, [Auto-Tuned singing] I think there might be something strange with us, maybe-- [Gumball puts his hand over Darwin's mouth]
Lucy Simian: What is wrong with you?
Gumball: [Auto-Tuned singing] If joy like this is wrong, then gosh darn it, I don't wanna feel right, bab-- [Darwin jams his fin against Gumball's cheek, shutting him up]
Lucy Simian: How could you be in such a good mood on a Monday morning in the middle of a surprise test?
[Gumball takes Darwin's fin and sticks it in the middle of the latter's face, making it look like he has a long nose]
Gumball: I don't understand it myself, miss, I just feel so hap-- [He sees Darwin's face with his fin on it, then points at him and laughs maniacally while coughing up a vibrant rainbow-colored fluid. Miss Simian starts to look shocked. The camera hits her face]
[In Principal Brown's office, he stands in front of a recording camera on a tripod, pretending to talk to a student in trouble]
Nigel Brown: I'm sorry, what did you say? Young man, I'll have you know that I'm the authority in this school. I'll let it slide for now this time. Or maybe I'll JUST-- [Pretends to fight, thrusting his fists at the camera. Miss Simian opens the door and Principal Brown freezes. Beat. Miss Simian closes the door and opens it again. This time, Principal Brown is in his chair and the camera is on his desk] Ah. Miss Simian. What can I do for you?
Lucy Simian: [Gets close to Principal Brown] There's a disease, spreading through the school.
Nigel Brown: What? What do you mean?
Lucy Simian: An outbreak. An outbreak of joy.
Nigel Brown: Rrright, now I'm not implying what you just said sounds crazy, it sounds more like: [Flails around in his chair, making weird noises, faces, and poses. Miss Simian looks unimpressed. He keeps doing it for a few more seconds before stopping] Actually, now that I said it out loud, it does sound a little crazy.
Lucy Simian: [Shrieks, frustrated] I'LL PROVE IT TO YOU! [She takes the camera and leaves]
Nigel Brown: Wait! Don't play the tape! [Softly] Uh, but if you do, for pity's sake, don't let it get on the Internet; people's lives have been ruined by things like that.
[A timestamp says 11:06 AM]
Lucy Simian: [Recording herself on the camera in the nurse's office] Dr. Simian's log. The subjects' state is now getting worse. The disease has distorted their faces into a hideous grimace. [She turns the camera to Gumball and Darwin, who have big smiles on their faces and dilated pupils]
Joan: There's nothing wrong with these kids, they're just smiling because they're happy. And for the record, you're not a doctor.
Lucy Simian: The happiness is the disease! [She stops recording and sets the camera down] And for the record, [Makes a long fart sound. She then turns to Gumball] All right, try and stop smiling.
Lucy Simian: [To the nurse] This should prove my point. [Gumball tries to pull his lower lip down so he looks unhappy] Try harder!
[Gumball tries harder, ripping his mouth off]
Gumball: Oh! I know! [Puts his mouth back upside down] There.
Lucy Simian: Hmm... [Gumball's face and mouth rotate, his expression ultimately going back to being smiley] See? This is proof that these children are diseased! It's scientific!
Joan: That was a lot of things ending in "ic", it was graphic, it was idiotic, it was sick, one could even say it was horrific, but it certainly wasn't scientific!
Darwin: I thought that was fin-flapping-tastic!
[A timestamp says 15:33 PM]
Lucy Simian: [Recording on the camera] The subjects seem to be getting worse, but still, no one takes me seriously.
Joan: Maybe that's because you're full of it.
Lucy Simian: Professor Simian, signing out.
Joan: You're not a profe—
[Static, then Miss Simian is shown shining a flashlight on Gumball, whose pupils are now very dilated and puppy dog-like]
Lucy Simian: Look at their eyes. They're all weird.
Joan: Maybe it's because you've been shining a flashlight into them for half an hour.
Lucy Simian: What, really? What time is it? [A timestamp says 16:06 PM, knocking down Miss Simian. She pants while getting back up and pushing down the timestamp] It's not just the flashlight. Look at those faces. [Waves at Darwin] Hello? Hello, is anyone there? [Switch to Darwin's point of view. Miss Simian is a live-action color-changing sock monkey and the nurse is a candy cane] Hello? Hello?
[Darwin turns to Gumball, who is live-action - except his eyes, brows, and whiskers. His mouth has human lips]
Gumball: Can you see what I see?
[Exit Darwin's point of view]
Darwin: The cuddle monkey says it wants a kiss.
[They both try to kiss Miss Simian]
Lucy Simian: No, no, no, no. Stop that. No, don't do that. What the-- [Pinches Gumball's lips] No familiarity! [Gumball and Darwin get up and Miss Simian backs up in front of the infirmary door] Oh, no, no. Get back. [She starts throwing jars at them] No! Aah! [Opens up a jar and throws the liquid at them]
Joan: [From the infirmary] What are you doing??
Lucy Simian: Throwing water at them.
[Gumball and Darwin squeak happily]
Joan: That's not water. It's cough syrup. AND THAT'S NOT HOW YOU TREAT STUDENTS!
[A timestamp says 16:36 PM. Gumball and Darwin are now in the infirmary, lying in beds, still smiling]
Lucy Simian: [Recording on the camera] I had no other choice but to put the subjects into quarantine. Their state is deteriorating. I will soon have proof of their infection. I also resolved my difference of opinion with the medical staff. [Ms. Markham pulls a piece of tape off of her mouth] Research supervisor Lucy Simian, signing out. [She stops recording]
Joan: [Standing near the hallway] "Supervisor"?? More like a super nut job! I'm not watching you torture children for being happy! I'm getting Principal Brown! [She leaves, closing the door]
Lucy Simian: Fine! I'll conduct my experiments without you!
Joan: You are not qualified to conduct experiments! [Miss Simian sets the camera down and mocks her] You're ugly enough without making faces!
Lucy Simian: Wait, what? How did she see that?
[Gumball and Darwin begin laughing hysterically. They cough up rainbows, stars, hearts, bunnies, and sparkles and they fall unconscious. The monitor flatlines, then becomes rainbow-colored and sparkly, playing a cheery tune]
Lucy Simian: Oh. My. Gosh. PROOOOOOF!
[Principal Brown and Ms. Markham are walking to the latter's office]
Nigel Brown: Like this? [He mocks Miss Simian]
Joan: No, more like this. [Mocks Miss Simian] And she's giving herself all these titles, I mean, what is a "Senior Mega-Consultant Supervirologist" anyway?
[They open the office door while Miss Simian dances]
Lucy Simian: IN YOUR FACE! Told you they were ill! Look at them!
[They all go to the door window of the infirmary and look in]
Nigel Brown: Oh, my goodness. The children! [Opens the door] Miss Simian, do you realize what this means??
Lucy Simian: [Looks worried] Oh, yes. It means [Smiles, points to him] YOU WERE WRONG and [Points to Ms. Markham] YOU WERE WRONG and I WAS RIGHT! [She dances again]
[As Miss Simian dances, Gumball and Darwin wake up as huggers and jump onto Principal Brown, knocking him down and infecting him with the Joy Virus. Rainbow fluid splatters onto the camera. Principal Brown gets up, laughing creepily. Miss Simian and Ms. Markham back away into the hall as Gumball, Darwin, and Principal Brown approach them. Miss Simian throws Ms. Markham into the infected, infecting her as well. Miss Simian enters her office, trying to close the door as they try to get in. She gets tickled a bit]
Lucy Simian: No! NO! [Slaps their hands away and closes the door. They break the door window, reaching in. Miss Simian screams and backs away toward her desk. She looks at the CCTV footage] Oh, no! It's spreading! IT'S SPREADING!
[On a surveillance camera, in the cafeteria, Gumball and Darwin enter and infect Alan and Bobert while Carmen and Tobias run away. Elsewhere, Rocky sprays something under his arm. The nurse jumps onto him, knocking him down]
Rocky: Aah! [Looks at the spray] Oh, it really works! [Becomes infected and laughs]
[In the hallway, Bobert shorts out the electrical power, darkening the school. On another surveillance camera, in the hallway, Alan infects the Eggheads, Juke, and Carmen. Touching her pops him, splattering rainbow fluid on the camera. Elsewhere in the hallway, Idaho runs away from Teri, Jamie, and Masami, who are infected and coming closer. Tobias steps forth]
Tobias: Lay-dees! Contaminate me. [They avoid him and go for Leslie instead, knocking him down and infecting him. Tobias sighs] The flower boy, typical.
[Elsewhere in the hallway, Bobert chases Anton while riding an electric floor scrubber. Gumball and Banana Joe chase Penny into the library, cornering her]
[Gumball infects Penny and kicks Banana Joe away. Penny laughs]
Lucy Simian: [From her office] It's infected everyone. [Inhales and in slow motion, says] NOOOOOOOOOOOOO! [In slow motion, she punches the file cabinet and hits it with a chair. She pulls out books from the bookcase and breaks off one of the shelves. She flips over the desk, but it lands exactly where it was. She tips everything off of it and flips it upside down. She knocks over a fan, causing a book to hit the cassette player’s button, which plays "Moonlight Sonata". Principal Brown, who is outside of the office trying to get in, pulls a sad face, gets cured, and runs away] The music! The music is the cure! [Looks up at the intercom] The PA system. It's our only chance.
[A timestamp says 18:20 PM. Miss Simian is slowly walking down an empty hallway, carrying the cassette player and keeping vigilant. Lockers are open and there is rainbow fluid on the wall and floor. She spots Anais faced in a corner, who sounds like she's crying]
Lucy Simian: Hey. Hey, kid. Kid, stop crying. I'm not infected. Just look at this face.
[Anais turns around - revealing she too has been infected. Laughing maniacally, she runs toward Miss Simian, who screams. Anais tumbles into a locker, which Miss Simian quickly closes]
Lucy Simian: No running in the halls. [Turns to another hallway filled with infected people and quickly hides] There's no other way. [Tries to pull a smiley face. She struggles, but eventually succeeds. She walks down the hallway, attempting to blend in] Haha, rainboowws! Peeaace!
Sussie: [Infected] Uuuhhhh!
Lucy Simian: Uuuhhh! [Banana Joe is strutting down the hallway, his peel down again. Miss Simian tries to resist, but gives in and fixes his peel] DRESS CODE!!!
[She catches everyone's attention and they start chasing her. She runs around the corner and encounters Teri, who tries to infect her. Miss Simian rips her arms off and continues running. As she dodges more infected people, she encounters the nurse, who also tries to infect her. Miss Simian sticks her to the wall and continues running. She runs into another hallway, encounters an infected Tina, and screams. Tina roars while backing her into the lockers. As some of her rainbow drool drips onto Miss Simian, Tina tries to infect her, but her arms are too short. Miss Simian gets past her and continues running. She turns another corner and runs into Tobias, who is not infected. She screams and he smiles awkwardly at her. Beat. She slams his locker door on him, knocking him down]
Tobias: Ow! What the — Ow!
Lucy Simian: I'm sorry, but it's your own fault, you're so multicolored, I thought you were one of them. [Continues running as everyone approaches from around the corner. She enters Principal Brown's office, puts the cassette player near the mic, and rewinds the tape] Come on, come on, come on, come on. [Everyone is now outside of the door] Hurry up. [The tape finishes rewinding] Yes. [As she's about to play the song, Gumball and Darwin emerge out of the darkness behind her] You gotta be kidding me. [She screams as everyone outside in the hall is shown. Fade to black]
[Static, then Gumball and Darwin are shown on top of Miss Simian, who's on the floor. The camera is recording]
Lucy Simian: [To the camera] The joy. It's starting to get to me. There's only one way to defeat it. [Cringes] You have to play a piece of music. [Inhales] I don't actually [Coughs] know what it's called, but it goes something like this. [Inhales] DAA DAA—Oh, sorry, I'm not much of a singer. DAA-- [Coughs] I'm sorry. [Coughs] I've let you down. This story...came to a happy...ENDING!
[She becomes infected and laughs maniacally before the video cuts to static. The camera's tape cuts to Principal Brown in his office, wielding and swinging a mop around. He stops for a few seconds to catch his breath, then continues. The episode ends.]