"Little Toa, you have not yet begun to see even the barest outlines of my plans. I have schemes within schemes that would boggle your feeble mind. You may counter one, but there are a thousand more of which you know nothing. Even my ... setbacks ... are planned for, and so I shall win in the end."
-Teridax to Vakama, Time Trap.
"The Plan" was a term used by members of the Brotherhood of Makuta when referring to Teridax's ultimate goal of overthrowing the Great Spirit. Not all Makuta were very enthusiastic about the Plan or its chances of success, but they went along with it because they knew Teridax would kill them if they didn't. Some, like Krika or Icarax, doubted the Plan would work, others, such as Gorast, were completely devoted to the Plan and while others remained neutral in the Plan's success like Bitil and Mutran.
Teridax first began to think that it might be possible to overthrow Mata Nui after the League of Six Kingdoms was defeated. Several centuries later, he attacked Mata Nui, making him fall unconscious and bringing on the Great Cataclysm. Just before the attack, Makuta placed the Matoran of Metru Nui into spheres that rendered them unconscious and caused them to lose their memories. His goal was to later awaken them and pretend to be their savior, after which they would hail him as their Great Spirit. But his plan was foiled by the Toa Metru, who rescued the Matoran and brought them to the island of Mata Nui.
Teridax then began harassing them with Rahi attacks, trying to keep them away from Metru Nui. But eventually, Teridax was defeated and the Matoran returned to their original home. Despite this, the Brotherhood's plan seems to still be in effect, and is entering its final stages.
On Voya Nui, Teridax took on Antidermis and tricked the Piraka into being competition for Toa. The Inika were supposed to grab the Ignika, but it plunged beneath the waves. Makuta took on Maxilos form, to aid the plan to completion, and slowed the Barraki from stopping the Toa Mahri. His Plan called for Mata Nui to die, so when he did, Makuta took his place. Matoro saved him, and the Toa Nuva brought him awake. Makuta now controlled the universe.
Teridax once compared his plan to the tactics of a Rahi called a Water Wraith. "Sometimes," he told his fellow Makuta, "the best way, the only way to win... is by losing." Teridax completed the Plan by allowing the Toa Nuva to reawaken Mata Nui, then he possessed Mata Nui's body (the Matoran Universe itself) and banished Mata Nui's mind and spirit into the Kanohi Ignika, which was then banished from the Matoran Universe. Almost all of the other Makuta were slain when Mata Nui woke up. The last remaining one died at the hands of the Order of Mata Nui.
Zaktan was the only one outside of the Brotherhood known to be aware of exactly what the Plan involved.