All the villains (except the Wolfy Kids) join forces to finally defeat the PJ Masks once and for all.
The museum has a new exhibit that features a Mega-Moonstone. Amaya, Connor, and Greg admire the poster that shows a picture of the powerful stone. Greg hopes that Luna Girl won't find out about the Mega-Moonstone, but just as he finishes saying that, Luna Girl's moths suddenly appear and shred the Mega-Moonstone poster. Connor proclaims that they can't let Luna Girl near the moonstone.
Once the PJ Masks transform and enter their HQ, Catboy asks PJ Robot if there is anything unusual going on in the city. PJ Robot shakes his head and checks the security camera footage on the PJ Picture Player, which shows a video of someone breaking into the museum. When PJ Robot pauses the video to show who the person was, the PJ Masks realize in shock that Armadylan broke into the museum. Quickly, Owlette presses the Owl-Glider icon, and the team flies off.
On their way to the museum, Owlette uses her Owl Eyes and spots Armadylan leaving the museum with the Mega-Moonstone. Owlette tells Catboy and Gekko that Armadylan has the moonstone, and Gekko asks if Armadylan stole it. Owlette tells him that it looks like it. Owlette then spots Night Ninja's Ninjalinos throwing Sticky Splats at Armadylan to pin him down to the ground. The PJ Masks then land the Owl-Glider and jump out. Owlette asks Armadylan what he's doing, and Armadylan says that he's "helping, what does it look like?". Gekko tells him that it looks like he is stealing the Moonstone. Night Ninja confirms this, and claims to have witnessed it himself, but Armadylan denies it. Catboy asks Armadylan if he didn't steal the Mega-Moonstone, and Armadylan says that he did steal it, but only to protect it from Night Ninja and Luna Girl, who he insists are working together. Catboy is still suspicious, so Armadylan tosses the Moonstone to the PJ Masks, saying that if they don't believe him, they should take it themselves. Unfortunately, before the PJ Masks could catch it, Luna Girl appears and catches the Mega-Moonstone instead. Catboy declares that they have to stop her. To break free, Armadylan decides to use a Thunder Thump. Night Ninja and his Ninjalinos run away, while Owlette tries to stop Armadylan. Owlette's efforts are unsuccessful, and Armadylan uses his Thunder Thump. Although he breaks free from the Sticky Splats, he also wrecks the Owl-Glider. Armadylan apologizes and says that he was trying to get unstuck. Gekko then asks Armadylan why he stole the Mega-Moonstone in the first place. Armadylan becomes frustrated and declares that he didn't and asks them why they don't believe him. He then stops arguing because he states there is no use in trying to convince them. He then leaves by Rolling Thunder, almost knocking over the PJ Masks. Owlette tries calling after him, but he doesn't return. She turns to the boys, and Catboy announces that they should go after Luna Girl and catch Armadylan later. He offers Gekko a piggyback ride, and they race off while Owlette flies overhead.
When out of Owlette's view, Luna Girl flies over to Romeo's Lab, which is in the park, and says "Oh, no! It's the PJ Masks!". When she gets no response, she repeats what she said. Romeo hears her this time and understands the signal. Robot then approaches Romeo and asks him if he will win this time. Romeo tells him that it was his plan, so of course they'll win. He just hopes that Luna Girl and Night Ninja won't mess it up. Night Ninja suddenly appears by smoke bomb and tells Romeo he heard that. Romeo then goes up to his lab and inserts the Mega-Moonstone into his lab to supercharge everything, the three villains sharing an evil laugh together as the lab powers up ominously. After that, Romeo starts handing out gadgets to the Ninjalinos and Night Ninja. Then, Night Ninja and Luna Girl leave to defeat the PJ Masks. What the villains don't realize is that Armadylan was watching them the whole time from behind a bush. He sees this as an opportunity to prove to the PJ Masks that he can be a good guy, and then leaves.
Meanwhile, high in the sky, Luna Girl gets Owlette to chase her. As she passes over the park, she gives a thumbs up to Night Ninja, who is down below. He orders one of his Ninjalinos to fire the Sniff-O-Whiff. The gadget is fired, only for it to be facing the wrong way so that it fires on Night Ninja and his Ninjalinos. They all gag and cough from the smell, and Night Ninja tells the Ninjalino that next time, he should make sure it's pointing the right way. Owlette then spots them and tells Catboy and Gekko to look out. Unfortunately, while she was distracted, Luna Girl sneaks up on her and fires her Luna Magnet, trapping Owlette in a Mega-Moon bubble. Down below, Catboy and Gekko realize Owlette is gone, but they can't focus on that for now because the Ninjalinos are attacking them with Sticky Splats. Catboy evades the ambush, but he soon comes upon the clearing where Romeo's lab is parked. Romeo sees Catboy and Gekko and he fires a Sticky Splat at them. Catboy skids to a stop, but by doing so, he throws Gekko from his back. This causes Gekko to get hit by the Sticky Splat and become trapped. Catboy declares that he will save him, but before he can, he gets hit directly by Romeo's Go-Slow Ray, which was fired by Night Ninja. To amuse his Ninjalinos, Night Ninja walks over to Catboy and steps on his tail. In slow motion, Catboy tells Night Ninja to cut it out, which causes the Ninjalinos to laugh. Romeo announces that it's his turn and fires the ice machine at Catboy.
After Catboy is trapped, Luna Girl arrives with a trapped Owlette. Romeo declares that he finally defeated the PJ Pests, but Luna Girl corrects him and tells him they finally defeated the PJ Pests. The villains then each say what they're going to do now that the PJ Masks are defeated. Luna Girl is going to take back her Lunar Crystal and go to the moon, Night Ninja will unlock the power of Mystery Mountain, and Romeo will take control of the PJ Masks Headquarters, reprogram PJ Robot, and take over the world. Gekko threatens the villains, and Catboy confirms what he said in slow motion. Romeo mocks them and tells Gekko that nobody can help them now. Suddenly, Armadylan appears and says that he wouldn't be too sure about that. The villains are taken by surprise, but they quickly come up with a plan to stop him. Romeo starts by distracting Armadylan and suggesting that he should join their team because he did steal the Mega-Moonstone from the museum. Armadylan announces that the only reason he stole it was to stop Luna Girl from getting her hands on it. As he says this, Luna Girl, on her Luna Board, circles around him, saying that thanks to him, they used the Moonstone to defeat the PJ Masks, and he could either join them or be the next hero to be defeated. Armadylan refuses the offer and says that he's not a villain. He prepares to do his Rolling Thunder, but Luna Girl fires her Luna Magnet and traps him in a Mega-Moon bubble. Then, Night Ninja fires the ice machine.
Unfortunately for the villains, Armadylan breaks out of the ice bubble and catches them by surprise once again. Night Ninja tries firing the Sniff-O-Whiff, but he fires the wrong way again. Romeo tries to fire the Stick Splats at Armadylan using his remote, but Armadylan dodges them and ends up crashing into Romeo's lab. The impact shakes the Moonstone out of the vehicle, and Owlette is freed. She then takes the Go-Slow Ray, places it into reverse, and fires it at Catboy. Catboy breaks free from the ice using his Super Cat Speed. Gekko is set free from the Stick Splat when Armadylan flicks it. As soon as Romeo, Luna Girl, and Night Ninja see all of the heroes free and ready to fight them, they retreat. After they leave, Armadylan gives the Mega-Moonstone to Gekko. While Gekko is admiring it, Catboy apologizes to Armadylan for thinking he stole the Moonstone. Armadylan accepts the apology and tells them that he was just trying to help. Owlette responds by saying that he was a hero when it counted, and the PJ Masks shout "PJ Masks, all shout 'hooray,' 'cause Armadylan saved the day!". Armadylan then declares that he's got to roll, and he leaves as the PJ Masks share a laugh, ending the episode.