The Island of Mata Nui was a tropical paradise, if one could ignore the mind-controlled fauna (called Rahi) that occasionally attacked the robotic inhabitants (Matoran) villages ("X"-Koro). After getting fed up with these attacks, one of the Matoran (named Takua) of the fiery village off Ta-Koro decided to go into the temple with some Toa Stones (magical gems that could turn Matoran into Toa, if said Matoran was destined to become one in such a way [Bionicle has a lot of circular logic, get used to it]) and try to become a Toa. He did not become a Toa, but he did summon a team of six such beings (the Toa Mata)
The Toa Mata then battled the Rahi and released the hold over them, after which they went searching for the one who started the attacks (named Makuta). They found his layer and did battle with him, eventually defeating him as well.