The Plan was Makuta Teridax's secret plot to overthrow Mata Nui and seize control of the Matoran universe.
A former Brotherhood of Makuta fortress held by the Dark Hunters during the Dark Hunter-Brotherhood of Makuta war hid a secret chamber which contained a chronicle of the Brotherhood's plans. The chamber had four stone walls inscribed with mysterious writing. Overall it seemed like nonsense, although to Zaktan it looked like notes for an experiment or calculations of some kind. The writing also contained three pictograms: a Great Hau symbolizing Mata Nui, the Kraahkan worn by Teridax, and an outline of Voya Nui. When Zaktan touched the Voya Nui symbol, the stones rearranged themselves so that the text became readable. The chamber was a chronicle written by the Brotherhood of Makuta, describing in detail the events leading up to the day the Great Spirit Mata Nui fell into an eternal sleep. It revealed that it was only the Makuta of Metru Nui who was responsible for Mata Nui's fall, and that the event that led it to it had actually taken place hundreds of years before sleep claimed Mata Nui. The walls also detailed Makuta's attempt to seize power in Metru Nui, prophecies about the great cataclysm, and a dire warning that the universe might end as a result of these events. There was a hastily scrawled warning about the possible coming of a Toa of Light. In one corner, there were newer carvings detailing how long it would take for Mata Nui to die of his injuries. At the bottom of these, one word was inscribed savagely in the stone: Ignition. It revealed that the Brotherhood had predicted that if the attempt to seize Metru Nui failed and the Matoran escaped, the villagers would eventually link up with Toa. These Toa were to be defeated if possible, but if that proved too difficult, they were to be allowed to believe they had won their ultimate battle, even if that meant the death of a Makuta. Once the Toa were blinded by their apparent success, the true plan would unfold. When it was done, Mata Nui would be allowed to reawaken, and a reign of darkness would begin beyond any Matoran's worst nightmares. The Brotherhood's plans made the Shadowed One's schemes look like idle fantasies. If they succeeded, there would be no more Toa, no more Dark Hunters, just shadows and death.
Zaktan was once able to read these secret records. Rather than tell anyone else, he decided to keep this information to himself for personal gain, determined somehow to insert himself into the grand scheme someday.
The Order of Mata Nui eventually became aware that Teridax had conceived of this idea since the Barraki rebellion. However, even 1,000 years after the Great Cataclysm, the Order had never learned how much of the successful attack on Mata Nui had been planned, and how much had been accidental. They also did not know whether Teridax had recruited his fellow Makuta before or after striking.
According to the Plan, Teridax was supposed to seize complete control of Metru Nui after the Great Cataclysm. The Matoran inhabitants were a particularly important prize.
The Toa Mata were key to the entire Plan.
According to Spiriah, if the Plan succeeded, the Makuta would rule the universe, their will enforced by Rahkshi. Anyone with the power to threaten their rule would be killed.
Makuta Teridax went to a secret place beneath the Metru Nui Coliseum to execute the final stages of his Plan. Zaktan had information about this.
Teridax later admitted that when he launched his plan to take over the universe, he did not think big enough. He thought he could be satisfied with seizing power in the city of Metru Nui. Doing that, he could control the flow of energy from the city's power plants to the rest of the universe. Not only this, but he would be able to decide when, or if, the Great Spirit would ever awaken again. Looking back at his setbacks in Metru Nui, Makuta realized that this was fate's way of telling him he was meant to rule more than just one city. The Plan began well enough; he captured and imprisoned Metru Nui's "pathetic" ruler, Turaga Dume, and took his place. He reprogrammed the Vahki to serve him. He hired Dark Hunters to eliminate Toa Lhikan and his teammates. He found a "foolish" mask maker, Vakama, and demanded he create a Mask of Time. Teridax patiently waited as his plans unfolded. One by one, Toa were made to disappear until only Lhikan remained. The Matoran of Metru Nui accepted Teridax as Dume, never once questioning his order to close the sea gates and isolate Metru Nui from the rest of the universe. The first moment when it began to go wrong was when Nidhiki and Krekka failed to prevent Lhikan from handing out Toa stones to six Matoran, who became the Toa Metru. Teridax had unleashed an intelligent plant menace of his own creation, the Morbuzakh, to drive the Matoran into panic. The Toa Metru used the Great Disks to destroy it. However, Teridax was able to use this to his advantage. The Toa Metru had temporarily used up their elemental powers in the fight and had not yet mastered their mask powers. When they were unable to show either to a crowd of assembled Matoran, Teridax branded them as frauds and ordered their arrest. The Vahki and Dark Hunters were only able to capture three, leaving Vakama, Nokama, and Matau free. Not willing to allow any delay in his plans, Teridax had some Vahki pursue the remaining Toa, while the rest captured the city's Matoran and put them into canisters where they would be forced into a deep sleep. Over time, they would lose their memories of how they had been imprisoned. Then Teridax would free them, and they would hail him as their rescuer, gladly giving him their loyalty and complete control of Metru Nui. Of course, this would take many centuries to happen, which is why Teridax wanted the Kanohi Vahi, the Mask of Time, which he could use to speed up time for the sleeping Matoran. The Toa Metru succeeded in freeing Turaga Lhikan and then dared to mount an attack on Teridax himself. Needing more power for the coming fight, Teridax absorbed the energies of the Metru Nui power plant, along with the minds and bodies of Nidhiki, Krekka, and Nivawk. He shapeshifted into a large, winged form, then challenged Vakama for possession of the Vahi, even as earthquakes crumbled the city around them. Vakama used the Vahi against Makuta to slow down time around him, but he was unable to control its power. Makuta was about to overcome Vakama when Lhikan surprisingly took a blast meant for Vakama to save his life, sacrificing his own in the process. The Toa Metru then attacked and trapped Teridax inside a prison of solid protodermis locked with a Toa seal. The Toa Metru mistakenly thought that Teridax was defeated and helpless, who reached out with his mind to a pair of his servants to arrive in Metru Nui and avenge his defeat.
From the beginning, Teridax knew that his actions might result in Mata Nui's death. He anticipated this, and admittedly might have hoped for it at some point. Teridax did not need Mata Nui dead, at least not at the start, but he believed that he had outlived his usefulness, too big, slow, and unfit for survival in the modern universe. Teridax rationalized that to take Mata Nui's rule, even his life, would be to show mercy to those lived there. For too long, they had been directionless. Now they would have purpose again: the greater glory of Makuta Teridax. As Teridax later reflected, despite his plotting and planning, trying to anticipate every contingency, he ultimately could not make this plan succeed without relying on others. Amazingly, he found himself relying on Toa and their sense of nobility, determination, and tenacity. He needed the Toa Mahri and Toa Nuva to play their parts in the drama he had crafted in order to bring down Mata Nui's long reign.
Mutran played a significant role in the creation of Makuta Teridax's plans.
The Toa Nuva and Takanuva escaped Karda Nui just as Teridax wished. Teridax had no intention of sharing the victory of the Brotherhood of Makuta with anyone else, so he purposely sent Antroz, Krika, etc. to Karda Nui because they were the Makuta most likely to challenge him someday. He believed it was far better to eliminate them without getting his hands dirty doing it himself, which the storm in Karda Nui accomplished nicely.
As Teridax expected, Mata Nui had died not long before his takeover, and the Toa Mahri had found some way to restore him to life, which he was counting on. In the microsecond between when the body was alive once more and Mata Nui's spirit returned to it, Teridax's own spirit entered it. Once he was in place, Mata Nui's consciousness was barred from residing in his own body, since two spirits could not occupy the same form. The Great Spirit rose, standing high above the unending sea, but it was Teridax, who had finally achieved his goal.
The inscriptions that Zaktan read hinted that Makuta needed to go to an unexplored place beneath the Coliseum of Metru Nui (see secret control room). It sounded like Makuta did not even know for certain what was there, or if the place actually existed, but was just guessing.
The other Makuta knew that Teridax would become the Great Spirit. Teridax promised that they would be his avatars in the universe.
Teridax could not have succeeded if Mata Nui had not died. Had his efforts on Voya Nui and Mahri Nui failed to delay the Toa long enough for Mata Nui to die, he would have had to amend the Plan, just as he did after the Toa Metru's victory in Metru Nui.
The fact that Mata Nui was asleep when brought back to life gave Teridax enough to time to replace Mata Nui's spirit.
Teridax wanted to strike down the other Makuta so there would be no one left with knowledge of how to attack the Great Spirit. "Trust" was not in the Makuta vocabulary. What the other Makuta expected was that Teridax would be busy doing Great Spirit-like things and would need other members of the Brotherhood to manage the day to day running of thing, which was a very practical idea. However, Teridax decided the threat to his safety outweighed the practical aspects of the idea.
Teridax's Plan did not end with taking over the Matoran universe. Rather, he had ambitions to traverse to other worlds and attempt domination on an even larger scale.
Up until his defeat by the Toa Metru, Teridax originally planned simply to take Mata Nui out of commission and take his place in the hearts of the Matoran. It was not until the Toa Metru beat him that he decided he could actually be Mata Nui.
Teridax did not plan for the Toa Mata's canisters to malfunction.