Following the previous events of the last segment, Connor, Amaya, and Greg are still amazed that their own city has a magical portal leading to the legendary Mystery Mountain. Since this is the case, there is a lot more that the kids would need to explore in the future. Therefore, the kids are in the library to find some books in order to learn some more information about the mountain for future references. However, as they reach the "Mountain Legends" section of the library, all of the books have disappeared from the shelves. All that is left are a couple of pieces of paper scattered around the section with Ninjalino footprints imprinted on them, plus some more footprints leading out of the library window. This evidence suggests that Night Ninja must be behind the disappearance of the books, which means that he must be planning on heading back to Mystery Mountain. Knowing that this is the case, the kids decide to stop him.
Arriving at headquarters, Catboy asks PJ Robot if there are any signs of Night Ninja. PJ Robot checks the surveillance cameras in the PJ Picture Player to look, and soon enough, there are Night Ninja and his Ninjalinos in the park, apparently reading the stolen books from the library. This observance strikes off as new to the PJ Masks, but knowing that they have the books, this means trouble from Night Ninja. Therefore, Catboy declares to take the Cat-Car, and the PJ Masks jump in and take off of headquarters, with PJ Robot staying behind to look after headquarters. However, as the team exits through the gate, one of the Ninjalinos pops out. Giving the all-clear sign, a couple of more Ninjalinos climb up headquarters and enter through Catboy's symbol doorway before it shuts. They slide into Catboy's base room, carefully sneak up on PJ Robot, who is cleaning up, and capture him just as he turns to see what is going on.
Meanwhile, the PJ Masks arrive to the park. However, as they look around, they see that Night Ninja and his Ninjalinos have disappeared. Suddenly, Catboy uses his Cat Ears, and is confused to hear PJ Robot in the distance. Wondering why PJ Robot is not at headquarters like he is supposed to, Owlette uses her Owl Eyes to see what is the problem. Soon enough, she sees that the Ninjalinos have captured him, and are taking him to the mountain portal. Buckling up, the PJ Masks quickly take off to the mountain portal in order to save their friend. A few moments later, after driving as quickly as they can, the team arrives at the portal. Unfortunately, when Catboy jumps out of the vehicle and tries to enter, it is too late; the wall closes. Wondering what to do to open the portal again, he starts doing the hand movement ritual. At first, he does it correctly, but then, he does the wrong gestures, and when he stops, the portal does not open up. With Catboy upset, Gekko walks towards the wall, and gestures to him that he will do it. Having memorized the ritual, Gekko does the hand gestures correctly, and ends it with drawing the infinity symbol. The portal opens up, and the PJ Masks enter.
On the other side of the mountain portal, the Ninjalinos are collapsing from exhaustion. As they take care of each other, Night Ninja paces around with a clock in his hand, wondering aloud what is taking so long for the PJ Masks to come. On cue, the PJ Masks exit the portal and demand Night Ninja to tell them what happened to PJ Robot and to release him this instant. Night Ninja explains that he trapped PJ Robot somewhere in the mountain. He agrees to the PJ Masks that he will be returned safely, but if he is going to be released to them, then they have to give him something in return, and that is the Ring of Ninjability. Upon hearing this, the PJ Masks complain. Night Ninja continues on, though, saying that the ring is locked in the pagoda, and whoever attains it and wears it will have their powers thrice multiplied. However, according to Owlette, the pagoda is also filled with booby traps, which explains the Ninjalinos' exhaustion. Therefore, the PJ Masks should go through instead, and not Night Ninja himself. Once they get through the traps and reach the pagoda, they must open the door by keeping this line in mind: "To open doors, solve this riddle; one by one the heroes come, but who finds the ring, not a single one." With that, Night Ninja orders the PJ Masks to move on if they ever want to see PJ Robot again, and they comply.
Meanwhile, somewhere in the depths of the mountain, PJ Robot is behind bars, being guarded by three of the Ninjalinos. Filled with sadness, he is playing sad harmonica music, which deeply saddens two of the Ninjalinos. However, the third Ninjalino is not at all amused by what is going on. He therefore shoos the Ninjalinos away, and gestures to PJ Robot to be quiet. Startled by his gesture, PJ Robot panics, quickly flies around, and crashes into a wall. However, upon crashing into it, he hears echoing throughout the cave. Knocking his fist into the wall, he starts thinking of an idea. A few moments later, everyone is climbing up the mountain. As they walk on the stairs, the PJ Masks are conversing with each other. As much as they do not want to give the ring to Night Ninja, they do not want to run the risk of losing PJ Robot for good. Gekko suggests that they should try to find the place where PJ Robot is trapped. Unfortunately, Catboy says that they cannot. Since there are lots of Ninjalinos around the place, they would just move him somewhere else, even if they did get closer to find him.
Soon, everyone arrives at the top of the mountain. One of the Ninjalinos unlocks the gate, and the PJ Masks go through. As soon as the gate closes, they find the pagoda where the ring is located. Gekko starts to walk along the path, wondering where are the booby traps located. Suddenly, he steps on one, and he is thrown up in the air. He lands on it, and it then throws him off the top of the mountain. Catboy summons his Super Cat Stripes, and he lassoes it on to Gekko's leg, enabling him to pull him down safely. Once Gekko is safe, thinking that the traps would not be a hassle if the PJ Masks are in the air, Owlette grabs the boys and flies towards the pagoda. However, the dragon statues sense the PJ Masks and shoot out dragon tongue splat. The team tries to dodge the tongues coming at them, but eventually, one of the tongues hits Owlette, and the PJ Masks roll back to the gates. Upon landing, they activate another booby trap, and the tile opens up below them. Luckily, Gekko grabs on to the ledge and pulls Catboy and Owlette out of the trap with his Super Gekko Muscles. Unfortunately, when the two land on another tile, it also opens up, and they fall in, until Catboy stops them from falling. Not wanting to know where the passages lead to, Owlette flies herself and Catboy out of there. Once the two are out, Gekko wonders if they are safe now. To answer the question, Catboy places one foot on another tile, and it sends the PJ Masks flying to the gates again.
Still in the cave, PJ Robot is trying hard to hear any echoes in the cave that could possibly lead him outside. Suddenly, he hears the PJ Masks screaming, and then Owlette's voice, saying that they should try again if they hope to save him. He becomes elated, and flies around in happiness. Going up to the wall, he quickly spins his hands around, and starts digging through the wall to follow the PJ Masks' voices.
Above PJ Robot, the PJ Masks are now determined to get to the pagoda. Catboy steps on the tiles, and jumps off of them quickly as they are activated. Soon, he steps on another tile, and multiple sticky vines are brought out. He uses his Super Cat Speed to dodge them, but he unknowingly leads them towards Owlette and Gekko, who are now in the grasp. Seeing this, Catboy quickly turns around and runs up the vine that has captured Gekko. He grabs his hand, and jumps off to the doors of the pagoda. Meanwhile, the vine that has captured Owlette is moving to the edge of the mountain and is preparing to drop her. Nodding at what to do, Catboy quickly spins in place and throws Gekko towards her. Gekko takes the vine off of her, and the two fly back to the entrance of the pagoda. As soon as they arrive, the vines fall back, and the booby traps are disabled. Now that they have made it, the team needs to figure out how to open the doors. Repeating the line from what Night Ninja had said earlier, the team decides to use their powers together to open them. With that said, the PJ Masks stand in front of the doors and chant, "Save PJ Robot," saying it faster each time as they keep repeating the phrase. After multiple chants, the PJ Masks activate their powers, and the pagoda doors open, revealing the golden magical ring. Catboy walks over to it and grabs it, awestruck by how amazing it looks. However, Owlette grabs the ring from him, and states that it is now time to save PJ Robot. Unbeknownst to them, as the team runs toward the gates, PJ Robot is still digging, following their voices in hopes to get outside.
The PJ Masks soon exit the gate. With Night Ninja waiting outside, Catboy tells him that they have the ring, and demand him to give back PJ Robot. Suddenly, the Ninjalinos jump out from the side of the mountain, and they throw sticky splats at them, trapping them into sticky splat boulders. Night Ninja gladly takes the ring from them and walks away. Gekko tries to remind him of his promise to give PJ Robot back, but the promise falls on deaf ears as the Ninjalinos push the PJ Masks into the gates and lock them in. Now realizing that they have been tricked, they are upset that they just went through all this trouble to save PJ Robot for nothing. However, one of the tiles start moving. After a few movements, the tile opens up, and PJ Robot flies out, much to the PJ Masks' excitement. Happy to be free, PJ Robot cuts through the sticky splat boulders, and everyone hugs each other, realizing that it was actually worth it going through all of this trouble because he saved them. In addition, Catboy states that because the team combined their powers togethers, there is no limit to what they could. That being said, he states that it is time to show the ninjas what they are actually made of.
Outside the gates, the Ninjalinos are gathered in a circle around Night Ninja. As Night Ninja places the ring on his finger, the Ninjalinos kneel down to get ready for his ultimate power. After a few seconds, though, Night Ninja does not feel anything. He tries to hit the ring and move it around in hopes of getting it to work, but nothing happens. Soon, the gates slam open, and the PJ Masks are standing before them, with PJ Robot back with them, ready to show Night Ninja the true power. Upon seeing them, Night Ninja quickly shakes the ring, trying hard to get it work. Seeing what is happening, though, remembering what the riddle had said earlier that whoever has the ring has powers multiplied by three, the PJ Masks realize that it actually meant that whoever has the ring has to use their powers together. The ring symbolizes that when everyone works together, the ultimate power is already obtained. With that, the PJ Masks take off to fight Night Ninja and his Ninjalinos together. The ninjas take off to fight the PJ Masks in response, but they are easily defeated by use of their powers together. Night Ninja starts running away, but PJ Robot stops him. Therefore, Night Ninja gets ready to fight him. However, PJ Robot takes out a couple of weapons that are installed inside him. Scared at his appearance, Night Ninja takes a couple of steps backwards until he trips on one of the books, falling down, and throwing the ring off his finger as it rolls towards the hole that PJ Robot made and falls down, presumably now becoming lost forever. As the gates close shut, Night Ninja demands the Ninjalinos to retrieve the ring. However, after knowing all the things that they have been through to get the ring, they run down the mountain. Night Ninja chases after them, promising them that he will try to make it work.
The PJ Masks share a laugh with each other, satisfied of the outcome of their mission. Gekko comments on how he has never been this happy to see someone pop out from a huge tile, while the team realizes that they actually have four times the power since they have PJ Robot with them now. Now that the ninjas are defeated, they can now take the books back to the library. They can properly check them out tomorrow, and finally read them for their future adventures on Mystery Mountain. With that, the PJ Masks cheer out their signature victory phrase, and the episode closes.
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The Ninjalinos became the second henchmen to be in the PJ Masks' HQ.