The Time Twins and General Machia take Kai, Nya, Ray, Maya, and the ailing Sensei Wu to the Boiling Sea, where Ray and Maya long ago hid the Reversal Time Blade in the sunken library of Hono Mizu. In order to traverse the harsh environment and retrieve the blade, Ray gives his children the Dragon Blade-the same two-bladed dagger Kai retrieved from the shop-and reveals that it has the power to merge the elements of fire and water to create a Fusion Dragon. Meanwhile, the other Ninja succeed in evacuating the various craftspeople captured from Ninjago, only for Zane to detect Cyrus Borg within a large dome. They successfully elude Commander Blunck and Commander Raggmunk, and then learn that the dome is the construction yard for Iron Doom, a monstrous creation which-thanks to Cyrus' enforced labor-has an engine that can utilize the power of all four Time Blades to travel through time, carrying an army of Vermillion with it.
After braving a rock slide, Geotomic Rock Monsters, and opening a large door using the Dragon Blade, Kai and Nya succeed in retrieving the Reversal Blade; however, upon returning to the airship Acronix and Krux refuse to let them use it to help Wu. A brief battle results in Ray being hit with the Time Punch and all five heroes being left to drop into the Boiling Sea, but Kai and Nya reform the fusion dragon to save them. Back at the swamp, the Ninja and Cyrus Borg-aided by Samurai X with a new color scheme-attempt to trigger a self-destruct feature Borg built into the Iron Doom, only to be halted by the Time Twins' return. The Time Twins and Vermillion then board the Iron Doom and open a vortex into which they are followed by Kai, Nya, and Wu after Ray and Maya jump off of the Fusion Dragon.