Trapped in the ocean's depths, mutated beyond recognition, the Toa must face their greatest challenges. Can they convince the Matoran who live in the watery city of Mahri Nui to trust them? Can they retreive the Mask of Life before the Barraki figure out how to control it? And will they be able to overcome the greatest challenge of all: the return of Makuta? What does this shadowy figure want from Matoro? And will Matoro end up betraying his friends before it's all over? Enter the unknown where friends can become enemies and enemies might be the only ones you can trust.