The episode starts with Connor, Amaya and Greg at the sports field, where Connor notices the equipment room is open. The room has been emptied of all the ropes and lanterns. On the floor Greg finds a note that implies Night Ninja took the stuff for his next plan, involving Mystery Mountain. The heroes decide to investigate.
That night, in HQ, their suspicions are confirmed; Night Ninja and three of his ninjalinos, including Teeny Weeny (who is suffering from a cold and is sneezing constantly), are on the mountain. The PJ Masks follow them with the PJ Rovers. On the mountain, An Yu also notices there are intruders and sets out to confront them. Meanwhile, Night Ninja and his ninjalinos are outside a cave that, according to a scroll, contains transforming splat. Night Ninja is afraid to enter it, so at first he sends Teeny Weeny instead, but changes his mind when he notices An Yu coming his way.
The PJ Masks arrive on the mountain, and Catboy uses his cat ears to track down Night Ninja. In the cave, Teeny accidently discovers the hidden room with the transforming splat, but falls into the splat when he sneezes. At first nothing seems wrong, but when he tries to leave the cave, another sneeze causes him to transform into a large, red splat monster. In this form, he scares Night Ninja and the other ninjalinos, and they flee from the cave, where the PJ Masks confront them. Teeny sneezes again and transforms back into his normal self. He also leaves the cave.
Night Ninja warns the PJ Masks that there is a monster in the cave, but they don’t believe him and investigate the cave, certain that Night Ninja is hiding something. Teeny follows them to get the scroll he accidently dropped back. He gets it, but another sneeze turns him back into the Splat Monster. He leaves the cave and outside transforms back to Teeny, making Night Ninja realize who the monster really is. Night Ninja is overjoyed to have his own splat monster, and orders him to attack the PJ Masks. The heroes try to flee, but the Splat Monster captures them with a giant splat ball. With the heroes trapped, Night Ninja plans to take over the city.
The PJ Masks try to escape by rolling the splat ball off the mountain, but this fails. An Yu finds them, and realizes Night Ninja found the secret room with the transforming splat. She uses her flute to free them and send the splat back to the secret room.
The PJ Masks ask An Yu to come to the city with them to stop the splat monster, but An Yu doesn’t want to leave the mountain. Reluctantly, the PJs leave without her. In the city, Night Ninja and the splat monster reach the library, but upon seeing his reflection in the fountain, the monster goes berserk and starts shooting splat everywhere. Night Ninja commands him to stop, but the monster no longer takes orders and attacks Night Ninja.
The PJ Masks also arrive, but before the monster can attack them, he sneezes and turns back into Teeny. Night Ninja quickly uses a leaf to tickle Teeny’s nose, causing him to sneeze and transform once more. The monster attacks both Night Ninja and the PJ Masks. On Mystery Mountain, An Yu’s flute informs her the PJ Masks are in trouble. Despite their best efforts, Gekko and Owlette get captured by the monster, while Night Ninja orders his ninjalinos to rob the museum of all its scrolls now that the heroes are distracted.
An Yu finally joins the fight, and tries to use her flute against the splat monster, but it turns out she has no power outside Mystery Mountain. The only effect is that the monster begins to dance. Night Ninja orders his ninjalinos to attack her, but An Yu manages to scare them off. She then resumes to play her flute, and while it can’t transform the monster back, it calms him down.
Catboy suggests they lure the monster to Mystery Mountain. It works, and while he follows An Yu, the monster also takes back all the splat projectiles he fired, thus freeing Gekko and Owlette. They take back the stolen scrolls. An Yu reaches Mystery Mountain, and with her powers restored, she takes back all the splat from the monster, transforming him back into Teeny. Night Ninja takes Teeny and disappears with him. An Yu and the PJ Masks celebrate their victory.
This episode features the first appearance of The Splat Monster.
Despite the Teeny Weeny Ninjalino becoming the Splat Monster, he was mostly referred by his original name than his monster form's name.