The museum has just discovered an ancient scroll about Mystery Mountain, and Connor, Amaya, and Greg are visiting to check it out. According to the legend written in the scroll, Connor explains that there is a gold magical ring at the top of Mystery Mountain. If the person were to wear it, he or she will gain ultimate "ninjability." In other words, according to Amaya, he or she will become the most powerful ninja ever. Just as the kids arrive to see the scroll, though, it is gone from its display. In addition, above the empty display is a couple of ropes that are hanging from the ceiling. From these observances, the kids realize that Night Ninja must have stolen the scroll. Seeing that he took it in order to find the mountain and get the ring to gain ultimate power, they decide to stop him.
At headquarters, Catboy asks PJ Robot if there are any signs of Night Ninja and his Ninjalinos. PJ Robot checks out the surveillance cameras, and it turns out that the Ninjalinos are running towards the back of the museum. He then asks the PJ Masks in his robot language, with the help of Gekko, if he can come on the mission with them as well, because he can help decipher the scroll. Knowing that it would be a great idea, the PJ Masks accept the offer, and Catboy declares to take the PJ Rovers. As the team takes off to the vehicles, Gekko asks PJ Robot to make sure that he does not get into any trouble, to which PJ Robot promises. The team then put on their helmets, start the engines of their rovers, and take off into town.
Behind the museum, Night Ninja and his Ninjalinos, along with the Teeny Weeny Ninjalino, are looking at the wall. Night Ninja asks the little Ninjalino that in order to get to the mountain, the scroll states that he must open up the portal, which is located on the wall in front of them. The little Ninjalino confirms this, and Night Ninja starts looking around, wondering how to open up the portal. After a few seconds, he becomes impatient and kicks the wall, injuring his foot in the process. The Ninjalinos laugh at this, but soon stop when Night Ninja angrily looks at them. He then turns angrily to the little Ninjalino and orders him to open up the portal himself. Nervously whimpering, he goes up to the wall and takes a look at it, then turns his attention to the ancient scroll. Meanwhile, the PJ Masks are looking from behind a building to see what is going on. PJ Robot takes a look as well, and soon focuses his attention on the little Ninjalino. He then begins to record him, as he starts doing a ritual that features multiple hand signs, ending it with drawing an infinity symbol with his two fingers. After he finishes, the wall opens up, and the portal to Mystery Mountain is revealed. Everyone is amazed at this sight, especially the PJ Masks, until Owlette loses her balance and falls down on top of Catboy and Gekko. Hearing the sound, Night Ninja realizes that the PJ Masks are here and turns around to tell them that he will be the one to go up the mountain and get the ring for himself. With that, he turns around and runs into the portal with the Ninjalinos and the little Ninjalino running after him. Then the portal closes, and the PJ Masks are left behind.
Catboy declares that they have to find a way to get to Mystery Mountain and get the ring first before Night Ninja does; otherwise, he will be unstoppable. Unfortunately, the PJ Masks do not know how, and the portal is closed. Luckily, PJ Robot moves in front of the team and approaches the wall. He plays the moment when the little Ninjalino does the hand movement ritual in front of it and soon draws the infinity symbol with it at the end. The portal opens up again, and the PJ Masks cheer happily. Catboy commends him, saying that it is great that he joined them for this mission, and declares to enter the portal. The PJ Masks soon get on their PJ Rovers, and they speed off inside the portal as the wall closes behind them.
After a few moments, the PJ Masks exit the portal and stop right where they are. Upon their arrival, they become awestruck as they look at the breathtaking surroundings around them. Catboy uses his Cat Ears, and he hears Night Ninja and the little Ninjalino conversing with each other about which way they are going to get to the ring. Hearing that the villains are just ahead, Catboy starts to drive after them. Owlette stops him, though, and explains that since the ring is at the top of the mountain, she can just fly up and retrieve it. With that in mind, she starts soaring high into the sky. Suddenly, thunder roars out in the distance, and multiple clouds from the distance cover up the mountain. Then, lightning starts flashing around Owlette, and after a few strikes, she falls down onto the seat of her PJ Rover. Embarrassed, she asks if there is a plan B, which is unfortunately what she was doing. Back to plan A, the PJ Masks start driving after the ninjas. They do not get too far, however, because as they make a turn, the PJ Rovers get stuck in a puddle of mountain sticky splat. With Night Ninja far ahead of them, the PJ Masks decide to leave their vehicles and follow them on foot. Luckily for them, Owlette sees with her Owl Eyes that they have just arrived at the edge of the forest, so the team quickly runs after them.
Arriving at the cave, the little Ninjalino tells Night Ninja that it is a shortcut. To make sure that's the case, Night Ninja tells him to check it out, to which he complies and runs inside. Suddenly, Night Ninja hears Catboy's voice. He hears that Catboy is worried of what kind of powers Night Ninja will have if he gets the ring. In response, Night Ninja looks over the ledge to see the PJ Masks, who are running to catch up with them, and sarcastically tells them that he hopes that they will get the ring first because once he gets it, they will never be able to defeat him again. The Ninjalinos then get out their sticky splats, and throw them at the PJ Masks. Gekko summons his Super Gekko Shields, and the others take cover behind him. While the splats are deflected, the team wonders if there is another way up this mountain, especially since this is a magical place. After a few moments, the little Ninjalino arrives above the ninjas and confirms them of the cave shortcut. With that in mind, Night Ninja asks one of the Ninjalinos to stay behind and guide the PJ Masks away from the top. The Ninjalino does not want to, but the others have run off in the cave. In addition, as soon as he turns around, the PJ Masks have run off.
The team soon approaches him. The Ninjalino prepares to fight them, but they only walk towards him and ask where did the other ninjas go. He starts pointing in different directions, but he then gets an idea. He decides to sits down and start to meditate. The PJ Masks are confused at this, but fortunately, PJ Robot gestures that the other ninjas must have gone up. Taking this into consideration, Catboy leaps up and scales the mountain, saying that the Ring of Ninjability will become his. Unfortunately, Night Ninja hears this and throws a couple of sticky splat vines that are hanging from the sides of the mountain at him to stop him. Catboy quickly dodges one, but the second one hits him, and he starts losing his balance. Seeing this, Owlette quickly flies up and pushes him back up the mountain. However, just as Catboy thanks her, the vine sticks to Owlette herself, and it starts swinging her around. Gekko quickly climbs up towards her, grabs her hand, and starts to pull her out. Meanwhile, along with PJ Robot, Catboy continues up the summit, and Owlette calls out that they will catch up as soon as they can.
Now at the top of the mountain, Night Ninja and his Ninjalinos approach a gate. Night Ninja attempts to open it, but it does not budge. He turns around to see the little Ninjalino, who is facepalming at what he is doing, and demands him to find out how to open the gate. Meanwhile, Catboy and PJ Robot arrive to the top of the mountain and become amazed. Knowing that the ring must be behind the gate, the two run up to Night Ninja and prepare to stop him. Seeing that the two are coming for him, Night Ninja pushes the Ninjalinos aside and attempts to open the gate by running towards it and slamming himself against it. It does not work, though, and he falls down on top of the Ninjalinos, knocking off the little Ninjalino off the top of the mountain. PJ Robot sees him hanging on the ledge, and comes over to him. He helps him up and places him down safely. Meanwhile, Catboy runs over to Night Ninja and confronts him. Night Ninja refuses to stand down, though, knowing that the ring is his. Now becoming a race between who is going to grab the ring first, the Ninjalinos help Night Ninja jump up to the top of the gates, while Catboy leaps along after him.
After climbing the gates, a big pagoda with a path lined with very big tiles and dragon statues comes into view. Catboy is amazed, but Night Ninja says that the view is amazing for a ninja's case, and jumps down onto the path. Catboy follows along, and notices that the ring must be in the pagoda. The two start going two it, but then begin to argue about who is getting the ring first. Just then, when Night Ninja steps on one of the large tiles, it moves. He and Catboy take a glance at each other, and then in front of them. Suddenly, the eyes on the dragon statues light up. They turn towards the two and shoot out dragon tongue splat at them. The boys jump up, and dodge them as they move towards the pagoda. Soon, one of the dragon tongue splats grabs Night Ninja's leg, and he is thrown around. Catboy tells him that he will save him, but then thinks of the ring, and proceeds to walk some more towards the pagoda. Suddenly, he steps on another tile, and it moves. It activates another dragon statue, which shoots out another dragon tongue splat. Catboy jumps up, though, and soars towards Night Ninja and grabs his hand to save him. Unfortunately, another dragon tongue splat pops out from another dragon statue, and it grabs his leg. Not knowing what to do now, the two just hold on to each other.
Meanwhile, Owlette and Gekko arrive at the gates. With PJ Robot and the little Ninjalino pointing to what is going on behind there, Gekko declares that they have to go in and help them. PJ Robot and the little Ninjalino open up the scroll and study it, hoping to get a clue on how to open the gate. Soon enough, they see three symbols on there, which is similar to the symbols on the lock. PJ Robot switches the lock to the right symbols according to the scroll, and the gate opens up, just in time for everyone to see Catboy and Night Ninja getting tugged. Upon seeing this situation, Owlette and Gekko quickly enter. Owlette flies up and grabs Night Ninja, while Gekko jumps on top of the dragon statues, avoiding the dragon tongues, and catches Catboy as he is pulled down. Gekko then tells everyone to leave. Catboy takes one last look at the pagoda, reluctant to leave, but he just sighs and joins along. Soon, everyone safely escapes the gates. Night Ninja gets angry at the little Ninjalino that he did not tell him of a easier way inside, and that PJ Robot had helped him figure it out. Suddenly, something starts to rumble, and the gates start to move. Night Ninja tries to get inside the gates again, but it closes. Catboy groans at this as well, but pushes this off, knowing that they will get the ring another time.
Suddenly, the mountain begins to rumble, and it starts shooting out mountain sticky splat. Everyone runs down the mountain and into the forest to take cover. Unfortunately, when they get there, the little Ninjalino points to the mountain. It is still shooting out mountain sticky splats, and this time, they are coming towards the forest. Seeing this happen, Night Ninja and his Ninjalinos run towards the portal. The PJ Masks notice this as well and quickly run to the PJ Rovers. With PJ Robot's help, they get their vehicles unstuck from the mountain sticky splat puddle, and drive off, PJ Robot in tow with Catboy. As the team passes by the ninjas, PJ Robot grabs the little Ninjalino's hand and pulls him in the portal, dropping the scroll as he enters. The scroll drops by Night Ninja, who stops and grabs it as the Ninjalinos enter in the portal as well. Catboy notices this, though, and uses his Super Cat Stripes to grab him and pull him through the portal as it closes. Night Ninja is angry at this, but promises that he will get the ring next time, and he jumps away with the Ninjalinos. The little Ninjalino follows along, but not before he gives a good-bye wave, and PJ Robot waves good-bye to him as well.
The PJ Masks are amazed at what they had saw tonight. Gekko comments on how Mystery Mountain is real, even though it was believed to be a legend, and Catboy starts to wonder how many secrets and powers is this mountain holding. Owlette concludes that even though they finished their trip there for tonight, it is certain that they will come back again, especially if Night Ninja is planning to do something there. With that, the PJ Masks cheer their signature victory phrase, and the episode ends.
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The "Teeny Weeny Ninjalino" makes its second appearance.
This marks the second time that the Teeny Weeny Ninjalino works with the PJ Masks