Aboard the Destiny's Bounty, Lloyd Garmadon is discussing with the ninja how he would attack a Serpentine. The ninja correct him excessively, and Lloyd asks Sensei Wu how he would attack a Serpentine. Sensei Wu responds with an ancient proverb and then tells the ninja that he has new suits for them. The ninja are happy to receive their ZX suits and are ready for a test run. Nya reports that Serpentine are terrorizing the Mega Monster Amusement Park. The ninja are more than ready to reply to the call.
Upon arriving, however, the ninja find that the Serpentine threat has been taken care of. Nya, who was at the scene, mentions to the ninja that a figure known as Samurai X had already arrived and handled the problem. The ninja are perplexed, and Master Wu, who comes out of a gift shop with cotton candy, tells them that the Samurai may help them better themselves through healthy competition. Interpreting their Master's advice as a hint that whoever catches the Samurai first deserves to be the Green Ninja, the ninja set off to find the mysterious warrior.
Kai, who is patrolling Ninjago City, drops Lloyd off at the Kiddie Arcade, much to Lloyd's protest. After Kai speeds off, Lloyd overhears the Serpentine preparing to head to the Lost City of Ouroboros. Lloyd dons a Serpentine disguise and follows the snakes on to a bus to the city. After a long drive, they reach the city, which is resting in the Sea of Sand. They all pile into a colosseum, where Pythor is announcing a Slither Pit. He tells the Serpentine that if he beats all of the Generals at once, he wants their complete, undivided loyalty. Skales, who is secretly supporting Pythor, gives him the stolen Sacred Flute to use against the Generals during the fight while he wears earplugs and pretends to moan in agony. Once Pythor emerges from the Slither Pit as the victor, he demands the Serpentine to bow to him. When Lloyd attempts to bow as well, however, he accidentally drops a part of his disguise, revealing himself to the tribes.
The ninja, back at the Destiny's Bounty, are complaining about their failed attempts to try and capture the Samurai. Sensei Wu comes in, noting that Lloyd is not present. Kai tries to claim that Cole was supposed to pick him up, but Cole places the blame on Jay. Sensei Wu, who is utterly afraid of what might have become of his nephew, tells the ninja to go find him.
They head over to the Arcade Kai had dropped Lloyd off at, and watch security camera footage from outside the building, spotting Lloyd putting on a Serpentine costume. Upon discovering his footprints outside, the ninja use their vehicles and begin to track down Lloyd. They come upon the gates of The Lost City of Ouroboros and slip past the guards. They come upon the border of the colosseum and spot Lloyd in a cage beside Pythor, but when they rush in to save him, they trigger a trap and are locked inside a cage as well, dropping their Golden Weapons. The ninja are led to the center of the colosseum, and Pythor announces another Slither Pit. Just then, Samurai X is also led out. The duel begins and the Samurai throws projectile weapons at them, but they dodge most of them. Pythor makes the game more interesting by tilting the stage and revealing spikes. As the ninja hang onto Samurai X's exo-suit, he tells them to keep the charade up and begins flying. However, the suit is too heavy and he ejects himself from the mech, allowing the ninja to escape safely, much to their anger.
As the exo-suit falls to the desert ground, Zane, Jay, and Cole fall as well, with the exception of Kai, who fell in a different direction. Kai spots Samurai X without his mask, revealing himself to be Nya. Nya explains to Kai that she was tired of how much focus was being placed on the ninja and she wanted to help out in her own way. Kai promises he won't tell the others, and Nya hands Kai the Golden Weapons she secured.
Back at the Destiny's Bounty, Kai explains to his teammates that Samurai X just handed him the Golden Weapons and left. As Sensei Wu leaves the ninja to go to bed, he hands Nya one of the Samurai's weapons, telling her knowingly that "iron sharpens iron, and sibling sharpens sibling," expanding upon what he had told the ninja earlier on.
This episode introduces the ZX ninja suits and the Lost City of Ouroboros. It is also the first episode where Samurai X's true identity is revealed to another character (although it is implied that Master Wu had known the truth sometime prior to Kai's discovery).