Timmy helps Mark Chang celebrate a Yugopotamian holiday in Dimmsdale. Only to find out that each day of F.L.A.R.G has their own weird meaning that will happen to those who celebrate it. If Timmy stops Mark Chang from celebrating F.L.A.R.G., his appendix will blow up and destroy planet Earth.
The episode starts with Mark Chang, an alien prince from the planet Yugopotamia. Mark was grumpy because he didn't get to celebrate F.L.A.R.G with his family. Right now, all Yugopotamians are celebrating F.L.A.R.G. on another planet. The only reason why Mark can't join them is because his fiancee, Princess Mandie, is waiting there to kill him. Just as Cosmo tries suggesting that he host F.L.A.R.G. on Earth, Timmy stops him, pointing out that a Yugopotamian having fun means misery for him. Unfortunately, he has no choice but to help Mark, as it is explained that his appendix would explode—with enough force to destroy the Earth—if he does not celebrate an authentic F.L.A.R.G.
Timmy wishes "To have the best F.L.A.R.G ever!" The holiday itself lasts for five days, each letter in F.L.A.R.G. representing what will be celebrated. 'F' stands for Fart. Mark lays an enormous, rank fart that results in Timmy, Cosmo and Wanda running for their lives, while several birds die. Timmy is left with the aroma. 'L' means 'You lie all day long', a rule that fustrates and embarrases Timmy. 'A' means Amnesia,which results in Timmy not remembering the best day ever, kissing Trixie, seeing Vicky and Mr. Crocker taken to jail, and getting a tattoo. 'R' means Ranksgiving, in which Mark has to eat food delicious to humans, which tastes disgusting to the Yugopotamian. As a result, after eating the food, Mark releases a huge vomit in Timmy's direction, who tries to avoid it by running across the world, even meeting Carly, a woman who he discovered tattooed her name on his chest during the 'best day ever'; but unfortunately, it caught up to him and he got hit at the very moment he reached home. His parents spend eighteen baths trying to scrub away the stench of alien farts and barf all over him.
On the last day of F.L.A.R.G., Timmy finds out that the "G" in F.L.A.R.G. means 'Goodbye' to the planet that the holiday was celebrated on. And by goodbye, Mark intends to destroy the Earth with a huge missile. This is the reason why the holiday is celebrated on a host planet, as Yugopotamia is the home world. Timmy attempts to wish the missile away, but his fairies can't interfere with a holiday. As it appears the Earth is doomed, Cosmo grabs Wanda and takes her into an 'escape hatch' he built. Eventually, Timmy manages to convince Mark that he cannot blow up Earth because it's now his home. After a "quick" moment of pondering, Mark realizes that he's right. Unfortunately, if his missile doesn't destroy the Earth, his appendix will. This leads Timmy to suggest sending it elsewhere (anywhere else Timmy states to Mark), which Mark agrees to and sends in new coordinates to somewhere else causing the missile to destroy a different world, preventing Earth's destruction. When Timmy asks Mark where he redirected the missile, Mark only smiles in response. It is soon revealed that the missile's new target was Fairy World. Emerging from the rubble, Jorgen von Strangle demands to know who's responsible for what just happened. Having just arrived, Cosmo blames Binky, who is instantly disintegrated, before taking Wanda into another escape hatch, resulting in crashing yet again. Binky notes "Oh FLARG!" as the episode ends.