The Toa Metru, heroes of the 2004 storyline.
2004 was a flashback year and told the stories about Toa Metru and the island Metru Nui. It was also the first year of the Bionicle Adventures story arc. The 2004 story was told in the following books: BIONICLE Adventures 1: Mystery of Metru Nui, BIONICLE Adventures 2: Trial By Fire, BIONICLE Adventures 3: The Darkness Below, BIONICLE Adventures 4: Legends of Metru Nui, (and the movie with the same title) BIONICLE Adventures 5: Voyage of Fear, and BIONICLE Adventures 6: Maze of Shadows.
Story
One thousand years before the 2001-2003 storylines took place, on the island of Metru Nui, a vicious sentient plant known as the Morbuzahk was trying to take over the city. Matoran were disappearing, and many places were becoming too dangerous to live in. But Toa Lhikan, the city's last living Toa, created six Toa Stones and delivered them to six Matoran: Vakama, Matau, Nokama, Whenua, Onewa, and Nuju, one from each of the six metru of the city. When he delivered Vakama's, two Dark Hunters named Nidhiki and Krekka attacked and kidnapped Lhikan. Shortly after this, Turaga Dume (little did Vakama know at the time that it was actually the Makuta posing as Turaga Dume) entered Vakama's forge and asked him to make a Mask of Time before the upcoming Akilini game. Using these Toa Stones at Ga-Metru's Great Temple, they became the Toa Metru. Toa Vakama told them of a vision he had: they must find six Matoran: Nuhrii, Orkahm, Vhisola, Tehutti, Ahkmou, and Ehrye, as well as the six Great Disks, the locations of which were known by the Matoran.
They split up and each went to their own metru to find the Matoran with the same element as them. All of the Matoran save Ahkmou (who was trying to hide, not be rescued) had been trapped by Nidhiki under something (or in Vhisola's case, hanging off the side of a building) and the Toa Metru managed to save them, some of them meeting with Vahki that wanted to capture and interrogate them. When they all met up again, they determined that one of them was a traitor: Ahkmou.
The Toa Metru and the Matoran of their elements split up into three groups: Vakama, Nuhrii, Onewa, and Ahkmou on one team, Whenua, Tehutti, Nuju, and Ehrye on another, and Nokama, Vhisola, Matau, and Orkahm on the last. After contending with Vahki, they found the Great Disks and went to destroy the Morbuzakh with them. The Toa Metru headed to the Great Furnace in Ta-Metru where they deduced the Morbuzahk's king root must be. The Matoran combined themselves into a Matoran Nui to allow the Toa Metru to get past the rapidly-sprouting Morbuzakh seeds in the Great Furnace, and they passed to fight the king root. After exhausting their elemental energies, they had to use the Great Disks, and they destroyed the king root.
They headed to the Coliseum to present themselves to Turaga Dume. But on their way, a Matoran named Nuparu informed them of a leak in the Archives of Onu-Metru. So the Toa Metru headed there to plug the leak, planning to present themselves to Dume after doing that. In the Archives, they encountered many Rahi and Rahkshi, all of them hostile. When they reached the Fikou web, a large tunnel system in the Archives, they split up and encountered a shapeshifting Rahi called a Krahka that attempted to become them and turn them against one another. Finally, they tricked her, and she attacked them using all of their forms combined into one. Using their powers together, they defeated her, but Vakama allowed her to escape.
Finally, the Toa could present themselves to Dume. But when they did, he did not believe that they are Toa. He puts them to the test by making them try to cross the Sea of Protodermis. They failed, and Dume sent Vahki after them. Onewa, Nuju, and Whenua were captured, while Vakama, Nokama, and Matau escaped. The Toa who escaped left through a chute, and found a Vahki Transport headed for Po-Metru. On it, they found strange pods. Vakama had another vision that told him Matoran were inside. In Po-Metru, they had an encounter with Nidhiki and Krekka, who they escaped using Nokama and Matau's newly-discovered mask powers of Illusion and Translation. Then they headed for the prison where the other Toa are being held.
Meanwhile, the other Toa had met a strange Turaga who had taught them how to use their mask powers. When the two groups of Toa met up, they had to fight more Vahki, and Lohrak. Finally, they found a Vahki transport, which they and the Turaga, who revealed himself to be Toa Lhikan as a Turaga, rode to the Coliseum. There, they witnessed the Matoran of Metru Nui being packed away into the pods they saw earlier, and Turaga Dume revealed himself to be Makuta. He absorbed Nidhiki and Krekka and Nivawk, his pet bird Rahi, into himself and transformed, absorbing the energy of Metru Nui and putting the Great Spirit Mata Nui to sleep. Vakama attempted to fight him, and discovered his mask power. He also tried to use the Mask of Time that he made from the Great Disks, but in the end, Turaga Lhikan had to sacrifice himself, and the Toa sealed Makuta in solid protodermis.
Taking six of the Matoran pods, the Toa Metru then headed away from Metru Nui and past the Great Barrier, heading for another island for the Matoran to live on. Eventually, they were attacked by giant sea Rahi, and saved by a Matoran named Mavrah, who was using the Kralhi, banished from Metru Nui after being replaced by Vahki, as his servants. The Kralhi attacked them, and a large battle ensued in which the path back to Metru Nui was sealed off, one of the Matoran pods was lost, and Mavrah drowned. They continued on, reaching a large island that they decided to bring the Matoran to. After exploring the island, they returned to Metru Nui, encountering several mutant beasts on the way.
Along the way, they encountered a Morbuzakh prototype called the Karzahni, named after the place in Matoran legend where bad workers were sent as punishment. It wanted them to get it energized protodermis to transform it into something strong enough to destroy Makuta, the one who abandoned it for being too smart. Being that Nokama was poisoned by an attack from the Rahi Nui and was dying, and the Karzahni had a cure, they had no choice but to comply with its wants. After an encounter with a being made of energized protodermis, they returned. The Karzahni was destroyed by the protodermis, but not before healing Nokama. So the Toa Metru began to make their way back home for the rest of the Matoran.