Metru Nui was once a prosperous city inhabited by the Matoran, ruled by Turaga Dume.
About 4000 years ago, the Dark Hunters plotted to take over Metru Nui. Vezok, Reidak, and Avak unleashed the Kanohi Dragon in a ploy intended to force Turaga Dume to allow the Dark Hunters to establish a base in the city. Instead, the Toa Mangai arrived, defeated the Kanohi Dragon, and took on the role of defending Metru Nui from future threats.
About 3000 years ago, Metru Nui became the battleground of the Toa/Dark Hunter War. The Toa Mangai ultimately emerged as the victors of the war, forcing the Dark Hunters to retreat after a massive army of Toa was summoned. Toa Nidhiki, who had betrayed the Toa Mangai to the Dark Hunters, was exiled from Metru Nui.
About 1000 years ago, the Makuta planned to put the Great Spirit Mata Nui to sleep and take over the Matoran Universe. To accomplish this plan, he kidnapped Dume and impersonated the Turaga, pretending to be the Matoran's benevolent ruler while he secretly turned the city into a dystopia, enforcing his rule with his Vahki squads. He allowed the Morbuzakh to take root in Ta-Metru's Great Furnace to terrorize the Matoran. He also hired Dark Hunters, including Nidhiki and Krekka, to assassinate the remaining Toa Mangai until Toa Lhikan was left as the sole survivor.
Before being captured by the Dark Hunters, Toa Lhikan sacrificed his Toa Energy to transform six Matoran into a new team of Toa, known as the Toa Metru. The Toa Metru defeated the Morbuzakh and saved the Onu-Metru Archives from being flooded. However, instead of being recognized as heroes, they were branded as traitors by the false Turaga Dume. Although the Toa Metru succeeded in rescuing Turaga Lhikan, they were too late to stop Makuta from putting Mata Nui and the Matoran to sleep. The Great Cataclysm occurred, wrecking much of Metru Nui in a massive earthquake. After defeating Makuta, the Toa Metru started evacuating the Matoran to a new island, which they named Mata Nui.
When the Toa Metru returned to evacuate the rest of the Matoran, they discovered that Metru Nui had been overrun by the Visorak hordes. They were captured and mutated into the Toa Hordika. With the help of the Rahaga, they were able to scatter the Visorak by defeating Sidorak and Roodaka, although the latter succeeded in freeing Makuta. They also found the mythical Keetongu, who cured their mutations. Using airships, the Toa Metru evacuated the entire Matoran population, bringing them to Mata Nui.
Since then, Turaga Dume and the Rahaga have been waiting in Metru Nui for the Matoran to return. In the meantime, they have been doing what they can to repair the city.
In 2010, during the Dino Attack, Dino Attack agent Zero went on a mission to Metru Nui. While exploring the island city, he found several Kanoka disks, including the Great Kanoka disks of Ta-Metru, Onu-Metru, and Le-Metru. He brought these disks back with him to LEGO City, where these disks transformed himself and Phantom into Toa.
Because of Zero's efforts on Mata Nui, some Matoran started to repopulate Metru Nui. Turaga Nuju and Matoro were among those who returned to Metru Nui.
A squad of Dino Attack agents led by Databoard and Voltage traveled to Metru Nui after responding to Louis's distress call from Mata Nui. During the mission, Snake was briefly infected by Mutant Dino Serum, and Turaga Nuju and Matoro helped them treat Snake. The Dino Attack agents traveled to the Knowledge Towers in Ko-Metru, where they salvaged technology to help Cobra recover from his amnesia, allowing him to remember his connections to ESG, the organization responsible for the Dino Attack. Cobra also remembered that ESG was developing super Mutant Dinos, and so the team returned to LEGO City to eliminate this threat.
Blade pursued Scoper to Metru Nui, where he was attacked by Robo-Blade. He emerged victorious over his robotic clone, destroying the robot with a Hyperdoken. However, the attack drained him of energy, and he passed out. When Blade woke up, he was greeted by Rahaga Norik at the Dino Resistance Force Headquarters. Norik explained that Blade's skill in defeating Robo-Blade would help defend Metru Nui from the Mutant Dinos.
However, two more Robo-Blades (named Byron and Langston) arrived and attacked. Blade used Teleportation Kanoka to send them to Ta-Metru, buying him and Norik time to prepare for battling Byron and Langston (who were busy battling Vahki). Norik unveiled the true purpose of the Dino Resistance Force Headquarters: building Exo-Force Battle Machines. Blade was given a Stealth Hunter built by the Matoran. Blade flew the Stealth Hunter back to LEGO City, unknowingly followed by Rahaga Bomonga in a Gate Defender. Piloting more Battle Machines, Norik and a cavalry of Matoran left Metru Nui to join the battle in LEGO City. Blade and Norik later returned to Metru Nui, where they combined the Stealth Hunter and Grand Titan into the Mountain Warrior, in hopes that the new Battle Machine would be able to defeat Cane's Anti-Dino Machine.
Metru Nui is comprised of six districts, each one representative of the elements Fire, Water, Air, Earth, Ice, or Stone. Ta-Metru is filled with furnaces and forges; Ga-Metru is a coastal school; Le-Metru is a transportation hub; Onu-Metru is home to the Archives, where Onu-Matoran study the past; Ko-Metru is home to the Knowledge Towers, where Ko-Matoran study the future; and Po-Metru is where sculptors practice their craft. Much of Metru Nui was wrecked in the Great Cataclysm, although it is slowly being rebuilt by Turaga Dume and the Rahaga.
During the Dino Attack, the Rahaga established the Dino Resistance Force Headquarters in Metru Nui. There, Rahaga Norik oversaw the creation of an army of Exo-Force Battle Machines, including the Stealth Hunter, Gate Defender, and Titan Tracker.
- Metru Nui was submitted to the Dino Attack RPG major locations list by BZP Noob #30000.
- When Dino Attack RPG allowed players to travel to Metru Nui, it was not yet known that the Matoran Universe (including Metru Nui) was located on the ocean moon Aqua Magna at this point in the story of official BIONICLE canon. Therefore, the RPG depicted Metru Nui as being located underground on LEGO Planet.