|“Hono Mizu was once a thriving port town that was consumed by an undersea volcano that buried the city leagues below an impossibly harsh environment. A combination of fire and water: The Boiling Sea.”|
|— Ray to Kai and Nya in "Out of the Fire and Into the Boiling Sea"|
The Boiling Sea is a dangerous body of water located somewhere that is presumably in the Endless Sea. It is a series of undersea volcanoes that engulfed the once thriving port town of Hono Mizu. The town is currently located in the largest volcano. Inside the town is an ancient library, which Ray and Maya used as a shelter to hide away the Reversal Time Blade under the orders of Wu after he discovered the weapon in the woods near the Monastery of Spinjitzu.
Following the defeat of Krux and Acronix after their powers of Time where absorbed by the Time Blades, the Time Blades were used by Wu and Garmadon to open a temporal vortex, which they used to dispose of the blades, but Krux and Acronix soon followed them, becoming lost in time. However, Krux would somehow immediately escape, landing in the woods below the monastery and fleeing, unaware of the Reversal Time Blade, which landed directly after he fled.
Shortly after this, Wu discovers the Time Blade in the woods. Knowing it was too powerful to keep, he ordered Ray and Maya to dispose of it within the Boiling Sea, knowing that only the current Elemental Masters of Fire and Water working together could reach the blade through a series of trials.
After Kai and Nya reunite with their parents, they explain to their children on what Krux and Acronix's master plan is; to create a time-traveling mech known as the Iron Doom, which would allow them and their Vermillion warriors to travel to any point in Time they please. After Nya realizes that the pause they felt earlier was the arrival of the third Time Blade, Kai realizes that if Krux and Acronix found the last one, their mech would be fully operational. However, Ray and Maya explain that Wu found it shorty after Krux and Acronix's defeat, where they were ordered by him to dispose of it within the Boiling Sea.
Maya then realizes that because their children are the current masters of Fire and Water, they could be used by the Hands of Time to retrieve the blade. Just before they could leave, they are confronted by Krux and Acronix, where they explain that Kai and Nya will recover the last blade for them, otherwise the dying Master Wu will perish.
Kai, Nya, Ray and Maya, along with Krux, Acronix, Machia and a dying Master Wu, use Machia's airship to reach the Boiling Sea. After Ray and Maya explain the history and what awaits them within the Boiling Sea, they use the Dragon Blade to create the Fusion Dragon, where they use it to traverse the Boiling Sea to reach the city of Hono Mizu.
After a series of trials and a fight with several Geotomic Rock Monsters, Kai and Nya are able to recover the Reversal Time Blade from within the library. As the who place begins to fall apart, they summon the Fusion Dragon to return to Machia's airship. However, Krux and Acronix reveal they had no intentions of letting them use it to save Wu, enabling a battle. In the end, Acronix strikes Ray with the Forward Time Blade and then pauses the group with the Pause Time Blade. As Krux claims the fourth blade, Acronix orders Machia to dispose of the group at the Boiling Sea. As the airship leaves without them, the effects of the Pause Time Blade wears off, causing everyone to plummet down towards the sea. Kai and Nya then summon the Fusion Dragon again and pick up the others before following Machia's airship back to the Vermillion Swamps.
Knowing that time was short and that Wu was soon to die, Nya got an idea and bailed from the Fusion Dragon, where she ran down to the woods and obtained the past Reversal Time Blade, knowing that since they were in the past, it hasn't been put in the Boiling Sea yet. With the blade, she was able to restore Wu back to normal. Following this, Nya and Wu explain to the distraught Kai about the recent turn of events with the discovery of the past Reversal Time Blade.
In order to recreate an established continuity, Wu threw the past Reversal Time Blade to his younger self, who was ordered by young Garmadon to dispose of it within the Boiling Sea.