With peace throughout Ninjago, Kai and the other Ninja fell out of training and instead spent their time playing video games. After a three hour marathon, they were interrupted by Wu who unplugged the TV, who encouraged them not to put off tomorrow what could be done that day and start training again to prepare for his brother's return to Ninjago. In response Kai reminded him of when they preformed the Tornado of Creation, but there Master informed them that they still needed to unlock their true potential.
At that moment, Nya rushed in bringing word that Garmadon was spotted in Jamanakai village. The out-of-shape Ninja then rushed out the door and down to the dragon pen where Kai mounted the Fire Dragon with the help of Nya.
Eager to get back in the fight, the Ninja raced each other to Jamanakai village, but upon arrival Kai argued with Jay who had arrived first. They were interrupted by the sound of evil laughter from the village and drawing their weapons, approached the village ready for battle. However, the Red Ninja was surprised to find that it was in fact Lloyd Garmadon, the dark lord's son, who was causing trouble by threatening to release the serpentine, an ancient race of snakes which once ruled Ninjago, unless they gave him candy. The Ninja tied him to the top of a sign and embarrassed him in front of the villagers as they mocked him by eating candy.
As they prepared to return to the monastery, Kai discovered the prophecy of the Green Ninja in Wu's bag which he had taken by mistake. Kai believed himself to be the Green Ninja as did the others so when they returned to the monastery, the four Spinjitzu masters challenged themselves to a tournament for the title. Kai faced off against Jay in the first round and won, thus moving on to the final round against Cole. However, the tournament ended when Kai lost control of the Sword of Fire and the Ninja were scolded by Master Wu.
Determined to be the Green Ninja, Kai and the others devoted themselves to training. They were soon ordered back to Jamanakai Village by Wu however as Lloyd had unleashed the Hypnobrai tribe of Serpentine. Kai once again mounted the Fire Dragon upon arriving at the village, met up with Nya. After discovering they would need the anti-venom from the General's staff to save the village, the Ninja split up and after a fight with the snakes, sent them retreating. After the villagers had been cured of their hypnotism, Kai presented Wu with the staff and apologized for their lazy behavior earlier. With a new threat now upon them, Kai and the others returned to the monastery determined to unlock the secrets of their Golden Weapons and to reach their True Potential.
One night, Kai ventured into the monastery kitchen for a midnight snack only to find that Zane had consumed everything but the cheese.
In the days that followed, Kai devoted himself to training at the monastery, though one time while he was practicing Spinjitzu, Zane iced the course over causing Kai to slip. He complained about Zane's strange behavior to Jay and Cole, only to be approached by Master Wu who noted that brothers are often different. They were then interrupted by the arrival of the Mailman who had a fan letter for Kai with him.
That night Kai and the others gathered in the monastery's dining hall and was surprised to find Zane emerge from the kitchen wearing a pink apron. As Zane failed to find it amusing, Cole dumped a plate of food on Kai's head and shortly after a food fight broke out.
The next day the Ninja followed Zane to Lloyd's Treehouse Fortress which the Zane had discovered the night before. Determined to destroy it before it became operational, Kai and the other Ninja snuck onto it and destroyed its supports. arrived Kai and the others discovered Zane was orphaned. The next day Kai and the others went to Lloyd's tree-house which acted as a Hypnobrai base. They started destroying it, but a hypnotized Cole turned on them. Before long Master Wu came riding in on Flame playing an ancient flute, which cancelled the Serpentine's powers. The group returned to the Monastery on Flame and discovered the Serpentine had set fire to it. Zane asked Shard put the fire out but, the Monastery was beyond repair. Kai and the others verbally attacked Zane but Master Wu caught them reminding them that Zane was their team mate and had them apologize. But before they could, Zane flew off on Shard to follow the Falcon. Kai and the others ate around a fire until Nya noticed Zane entering the camp, at which point he led them to their new home, the Destiny's Bounty.
After arriving on the Destiny's Bounty, Master Wu makes the Ninja clean the area. They do and put all the trash and junk in bin outside. However, later Jay's parents, Ed and Edna, arrive to see Jay there. After some time however, Ed and Edna leave Destiny's Bounty and go back to their home, located in a junkyard. However, the Serpentine are there and attack Ed and Edna. When the Ninja arrive, they find that they are too late and the Fangpyre had bitten Jay's parents and infected them with venom. Kai and the other Ninja, however, are ambushed by the Fangpyre, who have turned the whole junkyard into a snake infected area. Kai and the others defeat the snakes and leave with Jay's parents. After getting Fangtom's staff, Nya used it to cure Ed and Edna's venom.
After discussing who is the Green Ninja, Kai and the other Ninja are forced to train by Master Wu as punishment for being distracted and not doing their work. Kai, along with the other Ninja, are confused by Master Wu's riddle, "What is the best way to defeat an enemy?" Kai fails the riddle along with the other Ninja and assume combat is what Wu was talking about. After training and trying to solve the riddle, it is revealed that a boarding school has been attacked by Lloyd and an unknown Serpentine member, later revealed to be Pythor. The Destiny's Bounty takes them there, where they drop through the roof of the building on an anchor. Kai and the others chase the panicking Lloyd through the building until they reach the top of the building. As Lloyd and Pythor reach the roof, Kai and the Ninja apprehend Lloyd but Pythor escapes, stealing Lloyd's map. Master Wu, however, influences Lloyd to be good while reading a moral to him.
Kai discovers that all of his high scores have been replaced by Lloyd who at the time blamed it on Cole. However, Lloyd had messed up the other Ninja's hobbies as well. Master Wu reveals that he put Lloyd up to it to teach the Ninja "the destructive power of rumors." After this turn of events, Nya reveals that she has located the Constrictai and Venomari tombs. Kai and Jay take the Venomari tomb while Zane and Cole take the Constrictai tomb. As they arrive, Kai looks around, but is intoxicated with Venomari venom and begins to see elves and gingerbread men attacking him. Jay attempts to protect him while Kai is trembling from the visions. Cole and Zane arrive but are followed by Pythor and the Constrictai. As the two tribes surround them, the Ninja believe they will not survive the next attack. Kai hallucinates that a rope is in front of him, only for an actual rope to appear moments later. Suddenly, Samurai X arrives in a robotic mech and scares away the Serpentine. However, the Samurai knocks them unconscious while the Destiny's Bounty saves them from the Toxic Bogs. Kai's hallucinations then begin to fade.
Kai offered the other Ninja a bet: whoever finds the identity of Samurai X, they are the best and the Green Ninja. He attempts to capture the Samurai when he finds her in the Forest of Tranquility but the Samurai quickly captures him in a net. Later, when it's his turn to look after Lloyd, he drops him off at an arcade, then leaves, hoping to find the Samurai. After Lloyd has been captured by the snakes, Kai, Cole, Zane, and Jay go looking for him at The Lost City of Ouroboros. The Ninja get trapped in a cage that Pythor set up knowing they would come. Pythor puts them up to the challenge of battling Samurai X. The Ninja realize they are no match for the Samurai so they perform the Tornado of Creation and stop the Samurai. Worried the Ninja might win, Pythor tilts the platform, revealing a gigantic hole. When the Ninja and the Samurai are about to fall, the Samurai reveals she hasn't really been fighting with them and she will get them out of there, making her suit fly away with the Ninja clinging to it. When the suit crashes into the desert, Kai gets thrown farther away from the suit than Cole, Jay, and Zane. Seeing the Samurai also crash near him he goes to investigate. He sees the Samurai take off her head gear and recognizes it as Nya and promises to keep the secret.
As the Ninja continued to train, they became capable of impressive feats; Kai was able to run across a long path of red-hot coals with no ill effects. Later, he timed Zane as the latter sat underwater, expressing bewilderment as his white-clad ally remained beneath the waves for over ten minutes. After Zane finally returned to the ship, Kai and the others began speculating that they had reached their full potential, but decided to ask Master Wu before they jumped to conclusions.
When Kai and the others found Master Wu, he was packing his things in preparation for a journey. Nonetheless. Kai presented the Ninja's question, learning that they had yet to reach their True Potential. The Ninja soon learn of how Lord Garmadon came to be and why it is important to defeat him. Master Wu departs while Kai and the others put posters up for Lloyd, who had been captured by the Serpentine. Once the four notice the Falcon, they begin to follow it through Ninjago, hoping it will lead them to a Fangblade. After chasing it for miles, Kai and the others begin succumbing to exhaustion, telling Zane to continue following the Falcon into a snowy woodland. The Ninja eventually pick themselves up and find Zane to be horrified by something he's uncovered, learning that Zane is an android. Kai and the others still accept him and don't seem bothered by the revelation. After leaving the area, creatures referred to as Treehorns attack the Ninja and Kai is defeated by one of the creatures. The queen emerges, but is swiftly frozen by Zane, who gains his True Potential. The four of them leave the area and return to the Destiny's Bounty to discover Master Wu had not returned from his errands.
While watching Nya explain the venom process of a Fangpyre, Jay tries to impress her with men's cologne but finds out that Kai and the others pranked him by giving Jay perfume, which Nya is allergic to. The Ninja find that a Fangblade is at an amusement park and prepare to go there to acquire it. Jay attempts to go on a date with Nya at the park but Kai warns him not to, telling him "to get his priorities straight." The Ninja discover a ride that has Serpentine activity near it. The Ninja discover the Serpentine excavating the Fangblade and Kai declares a fight. Kai gains the upper hand in the fight along with the others but a mistake on Zane's part causes them to get frozen from one of his attacks. Kai and the others escape their confinement and Jay is revealed to have gained his True Potential while Nya is discovered by all the other Ninja to be Samurai X. Kai brags that he is the first to have found out and therefore should be the Green Ninja, but the other Ninja claim to not remember ever discussing that.
The Ninja, upset at losing the Fangblade, brainstorm for where the next Fangblade would be. Cole brings up that he knew about a trophy called the Blade Cup that seemed to be a Fangblade that his father once won at a talent show. The Ninja go to Cole's father and disguise themselves as dancers as to not cause suspicion around Cole's dad. They discover the Blade Cup is located at a talent show and enter it. However, the Serpentine have also discovered the Fangblade's location and disguise themselves as people in the audience, judges, and performers. When Kai and the others are ready to perform on stage, they began using various dance moves while the Serpentine attack them. Kai defends himself from them while Cole defeats the others using the Triple Tiger Sashay. The Ninja win the talent show and Cole's father is proud that they won the trophy without cheating as the Serpentine tried to do. Pythor attacks and nearly kills Cole's father but Cole protects him and gains his True Potential while the Serpentine manage to steal the Fangblade, making Kai the only Ninja left to find his True Potential.
Master Wu returns and surprises them by bringing Lord Garmadon, who has now grown four arms. Kai attacks Garmadon but Master Wu reveals that he is helping them search for Lloyd and order the Ninja to respect him during their temporary alliance. Kai gets more skeptical about Lord Garmadon and at one point overhears a conversation about the Green Ninja between Nya and Master Wu. Lord Garmadon discovers Kai eavesdropping and the two fight, only to be broken up by Nya and Master Wu. Later on, Kai searches for the Golden Weapons and discovers Garmadon is holding them. Thinking he is stealing them, Kai battles Garmadon and "wins." However, Garmadon reveals that he allowed himself to be defeated, as he was only fetching the weapons for the Ninja since they were heading over to their destination; this was confirmed when Master Wu told Kai that he was the one who told Garmadon to fetch them. Realizing this, the other Ninja scold Kai for his foolishness, leaving him humiliated. Despite the situation taken care of, Kai still retains his suspicion towards Garmadon.
After discovering the Fangblade is located at the Fire Temple, the Ninja go there to get it and rescue Lloyd from the Serpentine. Garmadon fights his way to Lloyd and rescues him but Pythor still has the Fangblade. In a desperate move, Kai impales the Fangblade to the cave wall with his sword, enraging Pythor. In retaliation, the Constrictai cause the lava to rise in the temple. Lloyd falls onto a rock but the Fangblade begins to fall into the lava. Garmadon attempts to save Lloyd again, but is unable to reach him and the other Ninja force him to leave the erupting volcano, much to his discomfort. Kai chooses to save Lloyd while the Fangblade falls into the pit below them. Kai gains his true potential and realizes that he wasn't meant to become the Green Ninja, but to protect him. With that, he transports himself and Lloyd out of the erupting volcano and back to the Destiny's Bounty. After reuniting with Lloyd, Garmadon genuinely thanks Kai for saving Lloyd, who is then revealed as the Green Ninja. Upon realizing this, Kai expresses an empathetic face, seeing how complicated it would be for Lord Garmadon and his son to be fighting each other in the future for the sake of Ninjago.
The episode begins with the Ninja cornering Spitta and forcing him to give them information on Pythor's whereabouts. Spitta tells them that they are days behind him. Back aboard Destiny's Bounty, Lloyd suggests that while Pythor is away, they should go to the Serpentine's inner sanctum to retrieve the other three Fangblades. The Ninja quickly find the snakes inner sanctum hidden inside the Constrictai Tomb but Lloyd and Lord Garmadon stay behind. In the tomb, the Ninja attempt to defeat the snakes, but to no avail. The Ninja are quickly captured and are forced to contact Lloyd for help.
Lloyd and Garmadon get into an argument, resulting in Garmadon leaving and heading to the underworld. and Lloyd gets a message from Nya asking for reinforcements. Lloyd goes on his first mission as the Green Ninja. Kai attempts to escape, but is quickly recaptured by Pythor who now has all four Fangblades. The Serpentine want to celebrate with a Slither Pit tournament, which Pythor agrees to. Lloyd arrives but comically falls down right in front of Pythor. Just before he is captured, however, Garmadon arrives with the Skeleton Army who battle the snakes. Lloyd frees the Ninja from their cage, and together they steal the four Fangblades and go back to Destiny's Bounty where Lord Garmadon leaves, making Lloyd upset. The Ninja are having a party for Lloyd getting the Fangblades, but little do they know that Pythor is outside their very window, plotting revenge.
Unaware that Pythor has snuck aboard the Destiny's Bounty, the Ninja plan to throw the Fangblades into the volcano. This fails when Master Wu nearly falls off the deck and is left clinging to the side, only for Kai and the others to save him. Pythor sneaks behind them but Lloyd warns them of his presence. Before Kai and the others can react, Pythor escapes on a Rattlecopter with the Fangblades and takes control over a tour bus. Kai fights off Serpentine on the top of the bus but is nearly knocked onto the road. Eventually Master Wu detaches the engine from the rest of the train and confronts Pythor, much to Kai's dismay and annoyance.
The bus crashes into the Lost City of Ouroboros, halting the Ninja's transportation. Fortunately, Lloyd arrives and takes them aboard the Destiny's Bounty. Just as Kai and the others attempt to save Master Wu, Pythor unleashes the Great Devourer.
After watching his mentor and Pythor get eaten by the Devourer, Kai quickly leads the Ninja and their allies back to the Destiny's Bounty, but the Devourer follows them and destroys their former home. As Jay begins to admit defeat, Kai inspires the team with some words of wisdom from Wu. The team then heads to Ed and Edna's junkyard on their vehicles. It is there that Kai reveals the blueprints for the Ultra Sonic Raider. While there they discover that the Great Devourer has followed them and they use the Tornado of Creation to build a machine out of all the scrap at the junkyard to defend themselves against it. The Ninja manage to drive it away from the area but Zane is injured during the conflict.
Realizing the serpent is heading to Ninjago City, Kai and the others follow it to its destination. While the creature begins destroying the city, the Ninja arrive and attack it. The fight is interrupted by the Ninja's Dragons returning as the Ultra Dragon. Kai and the others cheer their respective Dragon on until the Devourer gains the upside of the fight. Lord Garmadon arrives and alerts the Ninja that the only way to truly destroy the Devourer is to hit its weak spot with the Golden Weapons. Kai is reluctant to hand him the weapons but eventually agrees. Kai and the others lead the snake around the city, eventually trapping it. The Ninja watch as Garmadon jumps from a building and strikes the creature's head with the weapons, causing it to explode into a mass of green goo. Kai is annoyed that the Golden Weapons are no where to be seen, but rejoices when he discovers that Master Wu is alive. Victorious, the Ninja celebrate.