The episode starts off with the PJ Masks entering the pagoda to have some mountain flower tea with An Yu. They ask if they can have some fun and possibly have her show them the magical things hidden in the drawers. An Yu sternly disallows it, saying that fun is not allowed, and that there is a rule that dragon masters only can access the magic placed within them. She gives the team ten volumes of all the rules of Mystery Mountain that they need to follow.
Meanwhile, halfway down the mountain, Night Ninja and his Ninjalinos retrieve a special lost treasure from a secret dragon chamber inside the cave: the legendary statue of Munki-Gu, which can change the appearance of someone else. He uses it to turn himself into An Yu, and tells his Ninjalinos to give a fake invitation to the PJ Masks and get her out of the pagoda.
The next day, Connor, Amaya, and Greg are in Amaya's House reading the books that An Yu gave them to learn all the rules before visiting Mystery Mountain again, although feeling bored. When Connor opens another book, the fake invitation from Night Ninja falls out. To the kids’ surprise, it reads that An Yu is supposedly inviting them to a party. They transform later that night, and they drive over to the Mystery Mountain portal on their PJ Rovers.
“An Yu” comes out and greets the PJ Masks with some dance moves. “She” tells them that “she” changed “her” ways using the book that allegedly talks about having fun on Mystery Mountain. The only rule is that there are no rules at all. “An Yu” then dances through the portal. The PJ Masks are a bit concerned that An Yu is not acting like herself, but after some thought, they decide that they could get used to it, so they follow “An Yu” inside. They are unaware, though, that the An Yu they are with is actually Night Ninja in disguise. The real An Yu is being led outside of the pagoda by the Ninjalinos playing with one of the relics.
Traveling along the bamboo forest, the fake An Yu entices the PJ Masks to play with the rocks and sing as loudly as they can, despite the rules saying that they are not allowed to either play with the rocks or sing, especially in loud voices in the whispering bamboos. Gekko juggles a couple of rocks until one rolls away from his hands, though the fake An Yu says that anything can happen during their party on Mystery Mountain, and the four kids sing loudly as they traverse up the mountain.
On the mountain peak, the PJ Masks take on the challenge to skip all the booby traps and reach the pagoda safely. They quickly open the pagoda doors, and the fake An Yu runs over to the drawers and opens a couple of them, releasing some of the magic contained inside. The PJ Masks are concerned, knowing that the drawers are not to be opened no matter what. Their concerns are proven true when the real An Yu suddenly appears.
Night Ninja’s cover blown, he takes out the Munki-Gu statue to reveal how he transformed into An Yu. The Ninjalinos return next to him, and he uses the relic on them to take her form as well. The ninjas run outside the pagoda to cause some more trouble. An Yu chases after them, but she almost falls into one of the gaps the traps left behind. Owlette saves her, but one of the Ninjalinos takes her staff. Night Ninja takes it, and he uses the Munki-Gu statue on the dragon statues to trap the four kids. He then transforms himself and his Ninjalinos back to themselves, and they exit the gates, mocking the PJ Masks how they wanted to have some fun since An Yu is too serious.
The PJ Masks apologize to An Yu about not thinking clearly before breaking all the rules. The rules were meant to keep everyone safe. An Yu apologizes as well, saying that she should have not been too strict. Fortunately, she knows a way to get past the dragon splat tongues. The rule is to get past them, they have to dance slowly and seriously, but they should also have fun. That being said, the four heroes perform some dance moves slowly and seriously, while also having fun. The dragon statues soon calm down, and the dragon tongues recede.
The PJ Masks and An Yu exit the pagoda gates to find Night Ninja and his Ninjalinos celebrating their victory. Taking the information that the heroes are not invited to the party, Owlette blows the Ninjalinos away, while Gekko camouflages himself to take the Munki-Gu statue away from Night Ninja. Catboy also takes away An Yu’s staff from the villain, and An Yu sends him down the mountain.
The four heroes return inside the pagoda to put away the Munki-Gu statue. Before doing that, though, An Yu quickly steps out of the PJ Masks’ view. She transforms herself into Night Ninja, and scares the team as a prank. The PJ Masks are surprised, but amused since they see that An Yu is realizing that she can have some fun as well. She places the Munki-Gu statue inside one of the drawers, and the heroes proclaim the signature victory phrase.
Do not be fooled to break the rules.
Don't be too strict with others.
Sometimes you have to be serious and slow, but also have fun.