At Four Weapons Blacksmith, Kai and his sister Nya are hard at work crafting weapons when Sensei Wu comes by. The old man explains that he knew Kai and Nya's father, but Kai drives him off after an exchange of insults.
Moments later, a dark cloud appears over Ignacia, heralding the arrival of the Skulkin. Atop the Skull Truck, Nuckal begs to lead the attack, claiming that he's "dying to go down there" - beside him, his fellow General Kruncha irritably reminds him that they're already dead. Nevertheless, the Skulkin commander, Samukai, orders his troops to attack, and the Skull Truck races towards the blacksmith shop, surrounded by skeletal motorcycles.
Seeing the approaching skeletons, Nya wonders what's going on, but Kai simply tells her to stay in the shop while he confronts the invaders. Donning armor and taking up a sword, he challenges the Skulkin forces, doing surprisingly well despite the undead warriors' physical and numerical advantages. Wielding a bamboo staff, Nya joins in - despite Kai's warnings - and the two of them manage to fend off the Skulkin soldiers. Unnoticed, Kruncha and Nuckal sneak into the store, searching for something. After a scuffle with each other, the Generals find what they seek on the back of the shop's sign - a large piece of parchment.
With the soldiers defeated, Kai challenges Samukai himself, but the four-armed skeleton quickly overwhelms him. Kai is eventually knocked down and disarmed, but Sensei Wu intervenes before Samukai can deliver the finishing blow, knocking the Skulkin General around with a strange golden tornado of energy. After exchanging a few taunts, Samukai throws his daggers at a nearby water tower, forcing Sensei Wu to save Kai before the structure collapses. Retreating to the Skull Truck with his fellow Generals, Samukai tells Kruncha to "take the girl!" In response, Kruncha fires the truck's bone hand, snatching Nya as the skeletons retreat.
Kai thanks Sensei Wu for saving him, but demands to know what just happened. Sensei Wu explains that the Skulkin were acting on the orders of Lord Garmadon, the evil older brother of Sensei Wu himself. They have been battling for most of their lives, with Garmadon constantly seeking a means of escaping his prison in the Underworld. Wu then tells Kai that he sensed the "Fire" within him, which will allow Kai to save Nya - but first, he must train to become a Ninja. Confused but determined to save his sister, Kai agrees.
At the Monastery of Spinjitzu, Sensei Wu immediately puts Kai through the training course in the courtyard. After a few tries, Kai successfully completes the course, prompting Sensei Wu to tell him that his final test will be tomorrow. Satisfied with his progress, Kai prepares for bed, only to be ambushed by three mysteriousblackninja.