In anticipation of Garmadon's return, Sensei Wu seeks out four talented young teenagers to master the art of Spinjitzu and find the Golden Weapons before Garmadon and his Skulkin Army can claim them.
Wu recruits Kai, a young blacksmith and trains him to become a ninja at the Monastery of Spinjitzu. Nya, Kai's sister, is abducted by the skulkin and their leader, Samukai, Kai joins Wu's three other students, Jay, Zane, and Cole, in the hopes of rescuing her. The ninja team up to find the four Golden Weapons and save Nya.
They find the Scythe of Quakes at the Caves of Despair, learning spinjitzu along the way. They then find the Shurikens of Ice and the Nunchucks of Lightning, confronting each dragon in the process. That night Kai retrieves the Sword of Fire from the Fire Temple, and frees his sister, but is confronted by the shadow of Garmadon. Although Wu arrives at the temple and fights off his brother's shadow, the Skulkin ambush Cole, Jay and Zane and steal the other three Golden Weapons.
Sensei Wu decides to enter the Underworld with the Sword of Fire, believing that this is the only way to stop Garmadon. Samukai and the Skulkin army return to the Underworld with the Golden Weapons, and the ninja follow them, by riding on the backs of the guardian dragons.
In the Underworld, they defeat the Skulkin using the Tornado of Creation. Samukai fights Wu and takes the Sword of Fire, but the combined power of the golden weapons proves to be too much for him. A portal to the realm of madness is created in the process, which Garmadon uses to escape the Underworld.
Back in Ninjago, the ninja celebrate their temporary victory, with the expectation that Garmadon will soon return.