The legendary Toa Nuva arrive in the massive cave of Karda Nui in the midst of a furious sky conflict between Matoran villagers and the evil, bat-like members of the Brotherhood of Makuta. Shadow leeches have turned some villagers evil, forcing Matoran to battle Matoran to determine who will rule.
The Toa Nuva find themselves in Karda Nui, the core of the universe, on a mission to achieve their destiny. Although the life of the Great Spirit Mata Nui has been saved, he remains trapped in an endless sleep. Only the Toa Nuva can awaken him and restore Light to the universe.
The Toa are now armed with adaptive armor, weapons and masks, which alter themselves to suit whatever environment the heroes find themselves in. And they need any advantage they can get, for powerful, bat-like members of the Brotherhood of Makuta have launched an attack on the Matoran of Karda Nui. Already, the villains’ shadow leeches have turned many Matoran evil.
Both the Toa and their enemies must ally with Matoran if they hope to win this fight. The Toa discover the ability to forge a mental link with the villagers, gaining their knowledge of Karda Nui and unlocking new powers for the Matoran. The Makuta, blinded by the energies unleashed when Mata Nui's life was restored, must rely on the evil Matoran to act as their “eyes.” Those Matoran too gain access to new powers.
Believing they must retrieve the Mask of Life to complete their mission, the Toa Nuva decide to split up. Lewa, Kopaka and Pohatu remain to help the Matoran while Tahu, Gali and Onua head to the swamp far below. Lewa’s team challenges the Brotherhood of Makuta, but finds its enemies cannot be easily defeated.
Then help comes from an unexpected new ally – the Mask of Life, which has made a body for itself so it can live the life of a hero as “Toa Ignika.” With his power added to theirs, the Toa renew the fight.