Timmy is at the beach and he's bored silly. He's got all this merchandise from the Wet Willie movie (which stank) and yet he can't go diving because none of it will help him breathe underwater. Finally he realizes that all he has to do is make a wish, and poof, he's Timmy Turntrout! He, Cosmo, and Wanda set off to explore the deep, and everywhere they go, Timmy and Wanda receive a warm undersea welcome while Cosmo seems to upset the other fish. When the three happen upon the lost city of Atlantis, Timmy and Wanda learn why: Cosmo's been nicknamed "the accursed one" and hunted for centuries because he sank the city of Atlantis: nine times! Cosmo's arrested, put on trial, and immediately found guilty! Timmy appeals to Atlantis' leader, King Greg -- if he can show good cause why Atlanteans should be happy beneath the sea, Cosmo will be set free.
Timmy shows Greg the troubles of the surface world starting with waste dumping, to which Greg replied saying they would recycling; sewer pollution, to which Greg says they would adapt; social conflict (showing a bully trying to make Timmy's Dad leave the beach. Dad says he won't leave without Timmy - specially because Timmy's got the car keys. Timmy says he doesn't have the keys and we see two fish stealing Dad's car) and Greg says they would befriend the bully; seafood restaurants, to which Greg replies explaining their diet. Because it all fails, Timmy uses his last resource: showing Greg the Wet Willie movie and it works. Cosmo is no longer the accursed one and everybody was happy, until the giant squid previously called by Timmy to help them appears and destroys the city. Timmy is now the new accursed one. The fish who stole Mr. Turner's car offer them a ride to Clevelandlantis, which was previously suggested by Cosmo to avoid being discovered as the accursed one.
Timmy hasn't studied for a test in Mr. Crocker's class, so he wishes for a magical device that allows him to change places with anyone who comes in contact with it. Cosmo and Wanda don't have time to investigate; Momma Cosma's got the dreaded Nine Hour Flu and they must rush off to take care of her, so Timmy gets his wish. At school, he uses this buzzer device to change places with AJ, and lets the smart kid take his test for him. The plan backfires when Mr. Crocker tries to confiscate the device -- once he touches it, Timmy becomes Mr. Crocker and Mr. Crocker becomes Timmy! Now it's a race to Timmy's house, because if Mr. Crocker, as Timmy, gains access, he'll finally have the proof that fairies exist! Before any of them leaves, Timmy (in Crocker's body) must teach class and Crocker (in Timmy's body) is hung up on the mast by Francis, causing Crocker to understand next time a student complains about a wedgie. In classroom, Timmy sees it as a lifetime opportunity and orders each student to write a 500-word essay on the jerk he(Crocker) is. After that, he gives A.J. an F. Crocker takes the school bus to Timmy's house and Timmy takes Crocker's van. Timmy stops to have a tatoo, donate a kidney and attract some cops. He even parks at a towaway zone. Timmy tries to enter home but Mr. Turner, believing the "gorgeous" Denzel Crocker is hitting on Mrs. Turner, won't let him in. Crocker arrives and enters, only to be forced by Vicky to do chores and be bitten by Vicky's dog, Doidle. After that, Crocker is forced to do the homework he assigned Timmy to do and gives himself an F as force of habit. Timmy isn't able to enter as Crocker so he decides to use the device to swap bodies with somebody else. He changes bodies with his dad, who is happy for being "gorgeous". On his way to recover his body, Timmy swaps bodies with Mrs. Turner, Vicky, Doidle and finally recovers his own body. Now he was back on his body, he still had to make Crocker leave before Cosmo and Wanda come back so he uses a steak (Crocker is in a dog's body) to make Crocker jump through the window. The godparents are safe and there's still a big mess to undo like Mr. Turner being chased by the police and thinking it's because he's "gorgeous" and Crocker running away from the veterinarians who want to neuter him.