When an incident with The Bad Parent Hunter leaves Mr. and Mrs. Turner humiliated on national television, they vow to become better parents by becoming more strict. Annoyed at his much less lenient guardians, Timmy wishes to do the exact opposite of what his parents tell him to do. All is well until his parents tell him to "be good" before he goes off to school one day. Since he wished to always do the opposite, he becomes evil as a result.
Timmy needs help with his homework and he asks his parents, but his mom and dad are too distracted with their favorite show, The Bad Parent Hunter, so they gave Timmy a jar of deadly acid to shoo him off. The acid is so deadly it melts the door and allows Timmy to step outside through a hole. What the Turners didn't know is that their favorite program was being filmed live in front of their house! The Bad Parent Hunter observes Timmy melting his mailbox with the acid his parents gave him, and springs into action, responsibly opening the Turner's window first before jumping through and tying them up.
Later on, Timmy had invented a new word, "Funstructive!", which he used to describe his mayhem with the acid. Timmy's parents were upset about being humiliated on national television because of Timmy, and told him that for now on, they are gonna be good parents by being bossy. That includes telling Timmy to brush this tooth, do his homework, and take out the trash, all at the same time. Timmy is sick and tired of his parents telling him what to do and exhausted, collapses onto a garbage bag outside and complains to his godparents who were next to him disguised as flies. Wanda reminds Timmy that they are his parents, and it is not their job to make his life fun. Cosmo says its their job to do the opposite, just like wives, prompting Wanda to poof up a fly swatter and swat him.
Cosmo's words however gave Timmy an idea, and he wishes that he would do the opposite of whatever his parents told him. They wait a moment for Wanda to complain, but she decides to go along with the theme and do the opposite by not getting in the way of Timmy's foolish wish. The fairies raised their wands and the wish was made! Timmy asked to join his parents in watching a movie, and when Timmy was told to go to his room, he didn't. Instead he sat down, eats the popcorn, and enjoyed the movie with his parents who thought nothing of his disobedience.
The next day, Timmy is shown returning home from playing football with his friends, covered in dirt and mud. When his parents stop him at the front door and tells him to take a bath. The wish took effect and Timmy only decided to get dirtier, his parents not caring and proud of his stench. The next morning, Timmy wakes up in a filthy room, his stench was strong enough to green the Sun and cause the paint on his walls to peel. Wanda appeared with a gas mask, still refusing to say that this was a bad wish, and claimed she's still fun Wanda while a strand of her hair turned gray.
When Timmy was about to go school, he suggests to his parents that they say something like "Don't be late" or "Don't stay home from school", but instead his dad quickly says "Just be good", and they shut the door. Timmy says "Uh oh" and is transformed into Nega Timmy, a boy of pure evil with black hair, red eyes, sharp fangs, and a diabolical laugh. Wanda and Cosmo see this happen, Cosmo says that Wanda is probably going to say she was right now, but Wanda claims she still is enjoying the "fun wish" before all of her hair falls out.
At Timmy's school, Mr. Crocker was eyeing Chester, Timmy kicked the door open, walked to his teacher, and hissed. However Crocker thought this was an evil eye staring competition, but got thrown out by his evil student and into the mud. Crocker quickly deduces the work of Fairy Godparents, but decides he should probably find a line of work where he will never run into Nega Timmy again.
Wanda is becoming more worried, so she asked herself, "What's the worst that could happen?". At Timmy's house, his living room had been transformed into an evil lair, with a newly constructed "Super Deadly Evil Super Evil Laser" aimed toward the sky. Nega Timmy has an evil plan to shoot a satellite out of the sky and onto Mt. Dimmsdale, blowing the top off and causing it to erupt into a volcano, When the volcano goes active, the lava will flow into the Dimmsdale Dam, then with the water combined, the flood will head towards the Make-Up Factory, destroying it, and without make-up, social life will be destroyed in Dimmsdale, and the town will cease to exist.
During Nega Timmy's outlining of his plan, Cosmo is getting a photo booth prepared with Wanda for the exact moment she breaks down and says it is a bad wish. Wanda having heard what Timmy's plan is realizes that he is right as some people look terrible without makeup. According to her, nobody has ever seen Cosmo without his maeup. Cosmo then takes several pictures of her terrified looks (telling her "WORK THE TERROR, BABY!"). At this time, Mr. Crocker, now working as a door-to-door meat salesman, opened Nega Timmy's door and then immediately got thrown out again by the crazed boy into a thorny bush. Crocker says it is time to find another line of work.
The laser is now ready to fire, and Wanda finally gives in and pleads to Cosmo to help stop the wish. Cosmo says "Finally!" and scolds her for letting it go this far, but they had to disguise themselves as cats when suddenly Timmy's mom and dad entered saw the living room. They scold Nega Timmy and tell him to go to his room, but he doesn't go anyway and presses the fire button on the laser, destroying his room. When Mom tried to stop him and grounded him forever, Nega-Timmy told his father that the laser will also destroy Dinkleberg'shouse, which caused Mr. Turner to immediately change his mind and favor Nega Timmy.
The laser hit the satellite, causing it to plummet back to surface and onto the top of Mt. Dimmsdale, causing the dormant volcano to immediately erupt. At the nearby Dimmsdale Dam, Mr. Crocker, now employed as a tour guide, is currently showing people the dam. When the lava flow breaks the dam, it takes Crocker with it who manages to surf on a fragment on the dam. On a screen in Timmy's lair, Mrs. Turner sees the flood of water heading toward the make up factory and says that is where her husband gets all the stuff to make himself pretty, and that they had to do something. Mr. Turner tells his wife to give him a moment as he is aiming the laser at Dinkleberg's house.
Mrs. Turner protested, but her husband instead said to Nega Timmy to be "as evil as possible", reserving the effects of the wish and turning Nega Timmy back to normal. Timmy saw the trouble he had caused, and unwished everything, saving the Make-Up factory. Mr. Crocker fell into the make-up factory, and exited the building still alive, in one piece, and looking very pretty. Timmy apologized to his parents and says that he was going through an "evil phase" which is over for good.
While Timmy was talking to his parents, The Bad Parent Hunter overheard Timmy saying "You're both such good parents", which made him leave disappointed. Timmy's mom decided to get some ice-cream, his dad still wanted to blow up Dinkleberg's house with the laser but they pulled him away. Wanda says "That was too close" and Cosmo scolds her, saying that she should nag more often, Wanda mocks him the same way he mocked her causing Cosmo to pound the laser button in protest. The laser fires and hits Mr. Crocker, frying him. The Bad Parent Hunter comes over to Crocker and tells him, "You should put some make up on that". He then notes "Crikey!" as the episode irises in on him.