Creeps from the deep invade sunken Mahri Nui. The Mask of Life – descending from Voya Nui, floating above – draws evil Barraki as well as newly transformed Toa Mahri. The underwater village is soon at the center of a vicious battle between Barraki, Rahi, Toa, and guardians of the Pit.
Deep beneath the floating island of Voya Nui rests the sunken village of Mahri Nui. Connected by a stone cord to the island above, Mahri Nui is surrounded by air bubbles that allow its Matoran inhabitants to breathe normally.
But their existence is fragile. The Matoran must continually harvest air bubble from surrounding weeds, using the Rahi Hydruka to help. What’s more, their entire village is surrounded by The Pit, a seemingly bottomless chasm that holds the worst offenders of order.
The Pit was guarded by several warriors, including Hydraxon and Maxilos. Hydraxon perished a thousand years before during the Great Cataclysm when Mata Nui was put to sleep by Makuta. Robotic Maxilos and his pet Spinax hunt those who try to escape from The Pit… but Maxilos has lately become more than it seems.
The area is now ruled by the six most vicious of those imprisoned there. The Barraki were once powerful warlords, but their rebellion against Mata Nui was crushed and they were transformed and sent to The Pit as eternal punishment. Now that the guardians of the Pit are no longer in force, the Barraki engage in battle with other denizens of the deep as well as each other. Treachery is their pastime.
Into this maelstrom fell the Mask of Life. Attempting to protect itself from the forces arrayed against it, the Ignika created guardians for itself. One, a tiny creature named Gadunka, was enlarged to fearsome proportion. Another, a Matoran named Dekar, was transformed by the mask into the form of long-dead Hydraxon, who believes he was that warrior and must enforce order in The Pit.
Once the Barraki discover the existence of the mask, all other concerns are banished. They single-mindedly fight to control the Ignika, believing they will be able to transform themselves back into warlords and rule the land above once again.
The Toa Inika cannot allow this.
But first, their journey to the underwater depths is helped by the Ignika itself, which transforms the six heroes into water-breathing Toa Mahri. Surprised, they accept the change and discover the Matoran of Mahri Nui, who try to remain invisible to the awesome forces arrayed around them. After some initial suspicion, the Toa Mahri strike up a partnership with the Matoran of Mahri Nui.
Matoro makes a startling discovery: the robot Maxilos is actually Makuta in disguise! A strange understanding forms, although Matoro remains deeply suspicious of the dark one’s motives. Makuta tells Matoro that the stone cord connecting Mahri Nui to Voya Nui above must be destroyed, in order to set the stage for saving Mata Nui’s life and awakening. Matoro, doubtful, nevertheless convinces the rest of the Toa Mahri this is a necessary action.
Time has run out. Mata Nui is dead. The universe has three days before destruction.
The Barraki continue their attacks on the Toa Mahri and each other, ever grasping closer to the mask itself. Desperate battles, aided by a Toa from old, Lesovikk, finally give the Toa Mahri the opportunity to grab the mask and complete the evacuation of Voya Nui and Mahri Nui. A massive underwater battle comes to an end when the stone cord is destroyed by the Toa Mahri, and five of the six Toa make good their escape as Voya Nui comes crashing down upon them all.
But Matoro has other plans. He is the only one the Mask of Life allows to touch it without penalty. As the Ignika drags Matoro down through the gap in the bottom of the ocean, just before Voya Nui settles above them, Matoro falls into a massive cave. Knowing he is doomed, knowing Mata Nui is dead, Matoro puts on the Mask of Life.
The consciousness of the Mask, touched by the sacrifice made by Matoro, transforms the rest of the Toa Mahri into air-breathers again, then releases a massive burst of energy which restores the life of the Great Spirit Mata Nui. The mask plunges through the mist of the great cave, lost in the depths. Matoro is gone.
Mata Nui lives. And sleeps.