Hassle in the Castle is the fifteenth episode of Season 5.
Timmy wants to go into Cosmo and Wanda's fishbowl castle, but Wanda keeps telling him he can't. But when Cosmo and Wanda are asleep, Timmy snoops inside his godparents' fishbowl castle, but causes trouble after accidentally wishing back past godchildren from Cosmo and Wanda's Hall of Infamy. Timmy must recapture the godchildren before they can take their revenge on Cosmo and Wanda.
This synopsis needs improvement.
Timmy, Cosmo and Wanda are fish in the fishbowl, playing tag. They then decide to play hide-and-seek but there is nowhere to hide. Cosmo suggests hiding in their castle, mentioning that there are many rooms but stops talking after a glare from Wanda. Timmy wants to get inside Cosmo and Wanda's house. However, instead, they poof him to various other castles. His godparents act evasive, before they go into their castle for the night. It took a whole minute before Timmy decides to find out what's inside. He uses his shrinking suit to get down to fish size. However, the door demands a password. Timmy uses his imitation of Wanda's voice and said "Cosmo you idiot". Once inside, Timmy finds his godparents' castle big on the inside, with an arcade and a hall of his favorite foods. He also finds the Hall of Fame, composed of pictures of Cosmo and Wanda's favorite godchildren but he isn't in it. So he decides to take Cosmo's wand (password: "I should've married the monkey") to make the pictures come to life so that they could tell him what made them Cosmo and Wanda's best godchildren.
Meanwhile, both his godparents are sleeping, but Cosmo slightly heard Timmy. His female nickel Philip tells him it may be a nickel thief, to which Wanda points out that "we live in a magical castle underwater in a fish bowl in a ten year-old's room." Nonetheless, Cosmo decides to search the castle.
Timmy gets lost in the castle and accidentally stumbles into the Hall of Infamy, where he wishes three Cosmo and Wanda's old godchildren to life. He soon realizes his mistake when he is tricked into handing the wand over. Turns out, these godchildren, Pierre, Sammy and Maryann, were the worst godchildren ever, having made terrible wishes that made Cosmo and Wanda abandon him. The three of them fight over the wand, planning on exacting their revenge. Timmy manages to send Pierre and Sammy back into their portraits by slapping them with their respective frames. However, he has to chase Maryann before she could get Wanda's wand.
After a crisscross door scene, Timmy meets up with Cosmo. Together, they both race to the other wand. However, by the time they get there, Maryann turns them into ducks. Luckily, Wanda just woke up, calling Maryann a bad girl. Maryann accused her of abandoning her; Wanda says they didn't but points out that she abused their magic to take out Archduke Ferdinand and cause World War I. Maryann grabs the other wand but before she could eliminate Wanda, she uses a password ("Cosmo you dunderhead"). At that moment, a boxing glove comes slamming out of nowhere, knocking Maryann out cold, allowing her to be put back in her frame by Timmy and Cosmo.
As they put the pictures back in the Hall of Infamy, Timmy apologizes for sneaking into their castle and fears that he may be put in the same hall; Cosmo said that they didn't want him inside because the Hall of Timmy wasn't done yet because Timmy was the best godchild they have ever had. However, Timmy accidentally uses the wands to make the pictures of him come to life, who flood out of the pictures by the hundreds and pile onto Timmy and his godparents as the episode ends.