Five Days of F.L.A.R.G. is the eighth episode of Season 5.
Mark Chang is being particularly aggressive on the kickball field, and eventually Timmy learns why: it’s Yugopotamia’s biggest holiday, The Five Days of Flarg, and he’s missing it. He shows Timmy footage of the celebration, and Timmy notices that the revelry is happening on a planet that isn’t Yugopotamia. Mark explains that they always travel to a “host” planet, which gives Cosmo the brilliant idea of making Earth Mark’s host planet for his own Flarg celebration. Timmy doesn’t want any part of this, until Mark reveals that without the release the Five Days of Flarg provide, his appendix will burst.
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 frustrates and embarrasses Timmy. 'A' means Amnesia, which results in Timmy not remembering the best day ever, singing with Chip Skylark, eating ice cream with his friends, kissing Trixie, seeing Vicky and Mr. Crocker taken to jail, and getting a tattoo of a girl's name. '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 was the one whose name he tattooed 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 bathe him 18 times 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). Mark agrees to this and sends in new coordinates to somewhere else, causing the missile to destroy a different world, preventing Earth's destruction and ending F.L.A.R.G. on a good note for both Timmy and Mark. Mark even reveals that his appendix is no longer a threat to the Earth anymore.
Timmy asks Mark where he redirected the missile and 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 (presumably, to go back to Earth), resulting in crashing yet again. Binky notes "Oh FLARG!" as the episode ends.