Having been sent to the Realm of Oni and Dragons, Kai and the others are forced to struggle in surviving the threats of Dragons and hunters. They also teach a rapidly aging Wu some of his old lessons to better help him.
Marooned in the Realm of Oni and Dragons, the Ninja spend a full week in the realm trying to gather supplies. Kai is the most worked up over the situation, as seen when he criticizes Jay for not freaking out and states him being calm is unsettling to him. Kai and Zane spend most of their time building a radio in the hopes of communicating with the realm of Ninjago.
As Kai and Zane listen on the makeshift radio, they see that Jay has completely lost his grip on reality as he plays on his homemade game, which is really a piece of wood. After Zane tells them he has intercepted a transmission, they believe it's from Ninjago, however, the people on the other side state they are preparing something. Kai doesn't understand some phrases they uses and Zane translates the words to him by stating that they are talking about an ambush. Hearing this, Kai realizes they are talking about them and tells the others to run away.
As the trio run, Kai makes sure to keep a look out for the attackers before figures in the fog quickly grab each of the Ninja in chains and drag them off.
As they are held captive, Kai sees the Baron's throne (made from bones of the Ultra Dragon) and states he already hates him. When they were questioned over who else was with them, Zane tries to deny it, but Jet Jack shows them one of Wu's diapers. Kai spoke up by trying to pass them off as being Jay's as he tried to lie, saying he has a weak bladder, but they don't believe them. Iron Baron accuses them of being Oni while ignoring Kai's protests.
The captured Ninja are taken into The Pit which Jay notes is an arena where they will be attacked by something. Kai and Zane note that Jay is still not in his right mind and is useless in their plight. They find they are to fight a Dragon and are told by a hunter named Chew Toy to play dead.
During the fight, Kai goes through with playing dead and the Dragon, Slab, smells him but doesn't attack. Cole and Wu get their attention and use a lever to put a chain for them to trap the Dragon. Kai then alerts Jay that the Dragon would attack him but Zane freezes it, exposing their Elemental Power to all. Iron Baron realizes his mistake of thinking they were Oni and sees they are elemental masters. He then orders his followers to hunt them as Kai tells his friends to prepare for a brawl as the Hunters surround them.
Kai, Jay, and Zane are cornered by the Dragon Hunters and have no choice but to use their Elemental Power. Kai does an estimate on the foes before Zane corrects him on the exact number and he states Zane was better at counting. Kai asks Jay if he's all right, as Jay lists all of the events that have befallen him and has a breakdown; this display causes Zane to note to Kai that he is back to normal, which relieves him.
They use their Elemental Power to fend off the Hunters, when Heavy Metal jumps down with the Dragonbone Blade. They try to use their power on him, but as Zane points out, the blade attracts their Elemental Power. Now unable to use their power, they are chained up by Heavy Metal, and put in the back of the Dieselnaut.
Held in place by the Hunters with Vengestone, Iron Baron approaches them and admits his surprise at them not being Oni, but trained in the ways of the First Spinjitzu Master. Kai is shocked by his familiarity and asks if he knew him as Baron states he knows his legends and calls him the coward who ran away to Ninjago. Kai then asks if he knows how to get them back, to which Baron responds no, but he had no interest of helping them anyway. He then declares they are hunting Dragons and hopes that the catch he makes will be good.
He then turns on the machine and it releases a strange energy that triggers elemental reaction and the Ninja are subjected to a device which draws out their Elemental Power. Jay realizes that the Hunters are using them as a bait since their powers attract the Dragons while noting it tickles.
Soon after, a Wind Dragon is attracted to the Ninja. Meanwhile, Cole and Wu (who are disguised as Hunters) "accidentally" knock the other Ninja off the stand. They try to rescue them, but the Ninja are immediately put back into the back of the Dieselnaut by Daddy No Legs.
Now in the back of the vehicle, they are forced to sit back and watch as the Hunters reel in and eventually capture the Wind Dragon.
Cole promises to figure a way out of this situation for them, and the Dragons.
As he and his friends are caged by the Hunters, Kai sleeps before Jay wakes him up by telling him and Zane that he heard Lloyd's voice on the Hunters' radio. Kai questioned him if he has lost it again but after he explains they see he is still sane.
As the Ninja are forced to work, they wonder on what to do as Jay suggests building a Dragon and they call it out foolish but he tells them of his plan to trick Baron into thinking the Firstbourne was attacking so they and the Dragons can escape. The Ninja like his plan and get to work by secretly gathering supplies.
Later on, the Ninja are put to work by the Iron Baron, and Jay draws the blueprint for the fake dragon in hidden. While Kai and the others assemble parts and send them off to Cole and Wu, the two of them assemble the Dragon away from the camp.
Soon after, at night, Iron Baron gets ready to cook the Wind Dragon, and Kai and the others wait for Cole and Wu to "attack" the base with their creation. They are tied up, and watch the skies and then finally, with Wu controlling it, the fake dragon appears over the camp, and Zane and the others act like it is the real thing which spooks the Hunters.
While the Hunters scramble to their stations to capture it, Cole maneuvers to the others and frees them from their chains. They make their way out, and at the same time, Wu is shot out of the sky. At first the Ninja don't see it, and see the real Firstbourne in the sky and mistake it for Wu. When they realize it's the real thing, they freak out and attempt to flee.
Wu stops them and tells them that she is there to protect her Wind Dragon, but the Ninja assisted in capturing it. The only way for them to get out would be to save the Wind Dragon.
Kai and the others make their way to the Dragon, who is tied up, and get caught in a fight with the Dragon Hunters. Kai breaks off from the fight and frees the Wind Dragon. It helps the Ninja fend off the Hunters, and the Ninja also help the other imprisoned Dragons escape.
Kai and the Ninja flee the scene, and Iron Baron says they are going hunting again...for the Ninja.
The Ninja found a camp spot and sat around the fire before Wu woke up and they see that he has aged again and become a teenager overnight. When Jay commented it was bad, Kai mentioned they were teenagers and Jay called them "grown-up" teenagers.
Heavy Metal then finds the Ninjas' camp and makes her presence known, Heavy Metal threatens the Ninja with her weapon telling them they are her prisoners now before interrogating them on Wu truly being the son of the First Spinjitzu Master. Kai tells Wu not to reveal his heritage since it might be dangerous like their power but Metal fires an arrow at Cole after stating they heard Kai's mumbling. Kai watched as Wu revealed himself and Metal revealing themselves to be a woman named Faith who is on their side.
The Ninja haul the vehicle while Faith tells the legend of the First Spinjitzu Master. They were then approached by Daddy No Legs and Muzzle. They were able to tie them up and steal their vehicles to continue their mission.
When fighting over water with Kai, Faith slices a cactus in half so they could have enough. While they fight over the cactus, the Dragon Hunters caught up to them and they rode off. They made it to the canyons and managed to defeat them, but are unaware Jet Jack attached a tracking device on their vehicle.
The Ninja reached Oni territory and went in to look for the armor. They see the land was abandoned, but noticed the gates reveal a map to the Dragon Armor, which is located in Firstbourne's nest.
When the Ninja woke up, Faith handed them chain guns. Kai and the others are taught by her to use them to survive against the Dragons, which excites Kai.
Kai is tasked by Faith to demonstrate "catching" a rock to practice, and he fails as she realizes they all need some help. Kai says the first person to catch the rock wins.
While the other Ninja have a hard time using their chains, Zane gets it on the first try. He asks if he won, and Jay says they thought he was the judge. Wu joins the others in trying to successfully wrap their chain around the rock, and when the others get it except him, he gets frustrated. Kai gives him some confidence, and he returns to the others and wraps the chain around the rock.
A few moments later, they see the Stormbringer flying above them, and they attempt to prove themselves by catching their first Dragon. Zane and the others hold her with the chains, and Wu hops on top of her and rides her for a moment before she knocks him off.
Later on, Faith tells them that to get to the Armor, they must get past the mountain in front of them. They hear the other Dragon Hunters behind them, and realize they were being tracked. They fly through the mountains caves while they're chased, and Iron Baron says finds out that Wu is the son of the First Spinjitzu Master. He tells his Hunters to not go after Faith, but Wu instead. They all get through the cave, with some of the Hunter's men being left behind, and the Ninja realize there's no way to stop the Hunters unless they take Dieselnaut.
The Ninja dealt with the attack of Hunters and were soon able to hijack the Dieselnaut and escape but at the cost of Faith, who was captured by Baron.
At Wu's pleas, the Ninja go to rescue Faith from the Hunters. She berates them and warns them it was trap but the Dragon Hunters appear and capture them.
Kai is later tied to a stand with Cole and Zane, Kai shares banter with the others to keep their spirits up. He explains to Wu of why this is important and they share a laugh. He watches as Faith tells the other Hunters that Iron Baron lied to them about the Oni who have been gone from the realm for centuries and tells them that he possibly killed the Hunters to get Dragonbone Blade. After Iron Baron silences any growing rebellion by asking threatening them, an annoyed Baron hushes his prisoners by turning on the elemental device.
However, the torture causes Wu to regain all of his memories and slightly aged further into an older teenager. Kai watches as Wu breaks free of his bonds and begins to fight Wu with the Dragon Hunters but is knocked aside. He watches as Iron Baron negotiates with Wu who accepts the offer of leading him to the armor to return to Ninjago. Kai, Faith, and the Ninja tell him not to help Baron but he tells them to believe in him.
Kai is left to watch as Wu and Baron leave the area in search of the armor.
Kai and the others were still stuck on the wooden stakes. Above them, two vulture-like birds fly above them, and Kai asks if they are Zane's falcon. However, Zane remarks that his data picks up that they are a group of avian scavengers, that will pick them apart upon their demise. Faith confirms this, and Jay freaks out.
They notice that the other Hunters are arguing down below. While they're distracted, they try to rock their bodies side to side in order to knock the stakes over. The rocking seems to be working, and they cheer. As they do, Jet Jack asks them what they're doing, and she cuts them down.
After the Ninja are freed, they are skeptical of the Hunters' intentions and Kai tells them if they plan on eating them to eat Cole first since he is filled with cake as his friends are shocked by this. However, the Hunters reveal they want the Ninja to defeat Baron as they see Baron was only using them and will become even more dangerous with the armor.
They arrive at a bridge that leads to the armor, and find that it's too late, as the bridge has fallen.
Kai and the others return from the bridge to give the bad news to the other Hunters, and just then, Wu rides back the Firstbourne out of the nest without any chains, and discovers the Ninja, Faith, and the other Dragon Hunters, who had since turned on Baron in his absence and freed the Ninja.
As soon as he lands, he reveals Iron Baron's fate. The Ninja bow to him as Dragon Master, but Wu claims that it is he who should bow to them, as it was they who taught him everything since they arrived in the realm. Kai watches as Cole points out that their job isn't finished, Wu asks the Firstbourne if she will get him and his former students home. She agrees to do so, calling upon four other Dragons for the Ninja to ride. Kai gains a new Fire Dragon and is amazed at it.
With their work finally finished in the Realm of Oni and Dragons, Kai and the others ride the Dragons out of the realm, and begin their return to Ninjago.
Arriving in Ninjago through a portal forming in the air, Kai and the others are devastated by the city's state and start to look for their friends. They don't have to search for long as Nya contacts Kai and he is happy to hear his sister's voice and they make it in time to save them from Ultra Violet and her lackeys who flee after seeing the dragons. Kai then reunites with his other friends who are happy that he and the others are back however, Kai sees Skylor and learns from Lloyd that she was poisoned by Garmadon's power.
The Ninja continue fighting against the Colossus, using Vengestone Harpoons to try and incapacitate it. However, the chains are ineffective, and they eventually run out of them. Kai jumps onto the Colossus and begins binding the chains together even tighter.
The Ninja soon run out of chains and wonder what to do, before the Firstbourne returns and Wu fires his chain gun at the behemoth. The tides continues to turn in the Ninja's favor, as their allies escape prison and return to the city to help defeat the Colossus. Very soon, the Ninja and their allies incapacitate the Colossus using the chains, and it falls to the ground. Once Garmadon loses his power, the Colossus crumbles apart and the ninja are victorious. In Nya's armored van, Kai tends to Skylor who wakes up after her health is restored, assuring her they have won the battle.
They then have the Sons of Garmadon arrested before Lloyd arrives with his defeated father and the ninja celebrate his restored power while Garmadon is arrested.
Several days later, the Ninja clean up the Royal Palace and complain since they saved the city before Wu (an old man again) tells them there's trouble.
The Ninja run out of the palace and find a crowd celebrating their victory. During this time, Kai and Skylor hug before becoming a couple.