The movie revolves around the latter half of the 2003 storyline, on the island called Mata Nui. The spirit protecting the island was put into a deep sleep, and only the Mask of Light, in the hands of two Matoran who are assisted by six Toa, can return it to the Toa of Light, who is predicted to defeat the Makuta.
The direct-to-video sales made Mask of Light one of the top selling DVDs of 2003 in the United States, and has helped the development of three more movies in the franchise. It was praised for its visual effects and sound direction, but thought to be average in its storyline and character development. The movie was followed by the two prequels Bionicle 2: Legends of Metru Nui, Bionicle 3: Web of Shadows and one sequel Bionicle: The Legend Reborn.
The movie premiered on Cartoon Network's Toonami block on April 17, 2004, at 7:30 PM, as part of the premiere of the Saturday night Toonami block. A preview for Bionicle 2: Legends of Metru Nui was shown at the end of the premiere on Toonami. Several hours before the movie aired, the Bionicle website posted a question for the fans, asking them to quote Vakama's speech during the sneak peek in order to obtain special content on the website's Kanoka Club page.
In 2003, when the movie was released, Toonami held the "Search for the Seventh Toa on Cartoon Network" Sweepstakes, a call-in sweepstakes which gave 1 grand prize winner a Platinum Mask of Light, among other prizes.
The story begins with Turaga Vakama retelling the original BIONICLE legend; long ago, the Great Spirit Mata Nui descended from the heavens to an island paradise, bringing with him the islanders known as the Matoran. As they were without purpose from the start, the Great Spirit united them with three virtues: unity, duty and destiny. In gratitude of these gifts, the Matoran named their island in his honour. Yet their peace and harmony didn't last, as Mata Nui's envious brother, Makuta, betrayed him by casting a spell over Mata Nui, causing him to fall into a deep slumber. With his brother struck down, Makuta had free rein to spread his evil across the island.
The story then shifts to the fortress village of Ta-Koro, located in a lake of lava, as a Ta-Matoran named Jaller is looking for his Kolhii (a lacrosse-like game) teammate, Takua. He soon finds him inspecting a warning totem next to a lava flow. Just before they leave, Takua picks up the totem, unwittingly setting off a booby trap, and dropping the artifact into the lava, revealing a Kanohi mask that was embedded in the totem. At that moment, a wave of lava begins to rush through the chamber at Takua, who throws the mask to Jaller and tries to use a lavaboard to cross the lava. Just as he is about to be killed, he is saved by Tahu, the Toa of Fire. Takua and Jaller then rush to the Kolhii field, and Jaller puts the mask in his pack.
After the match ends (during which Takua tried using a new move he had practiced before the match, with no success), the mask Takua found falls out of Jaller's pack, shining when in contact with Takua, who tilts it towards Jaller. The Turaga reveal that this is the Mask of Light which is to be worn by a legendary Seventh Toa that will defeat the Makuta. The Mask of Light will lead its herald to the Seventh Toa, but Takua, who was thought to be the Herald, is unwilling to admit it so Jaller is mistakenly believed to be so. Unfortunately for Takua, he still has to come along to chronicle Jaller’s quest. Pohatu, the Toa of Stone, leaves to spread word about the Seventh Toa, while Gali, the Toa of Water, departs to ponder the situation, noticing a seventh spirit star in the sky. The two Matoran leave Ta-Koro the next day and follow the mask’s light to the jungle region of Le-Wahi.
Deep beneath Mata Nui, Makuta is fully aware of what is happening above. Speaking to a large statue of a Kanohi Hau, which he refers to as his brother, Mata Nui, he decides to release three of his spawns of shadow, the Rahkshi, to find and destroy the Herald. On the surface, Gali is meditating at Kini-Nui, the Great Temple, when the Rahkshi burst from the center of the temple and attack her. She narrowly escapes by hiding in the river. Realizing the Rahkshi are headed for Ta-Koro, she hurries there to warn the villagers. The Rahkshi burst through the walls of Ta-Koro, using their powers of fragmentation (Panrahk), disintegration (Guurahk), and poison (Lerahk) to eventually destroy the village. Fortunately, all the Matoran escape unharmed. In the fight, Tahu's mask is accidentally damaged by Gali when she tried to get Lerahk off of him, leaving a sickly green scratch. However, he is more concerned about his village's destruction after he and Gali abandoned the place by lavasurfing.
Meanwhile, Takua and Jaller are traveling through the jungle of Le-Wahi, going where the Mask of Light shows them to go. Attacked by a Graalok ash bear, they are rescued by Lewa, the Toa of Air. He gives them a faster way of traveling, a Gukko bird, which quickly carries them to the frozen region of Ko-Wahi. Upon their arrival, they learn of Ta-Koro's destruction, and Lewa flies to Ta-Wahi to learn more, leaving Takua and Jaller behind. They get caught in a blizzard where they encounter a squad of Bohrok that are frozen, and run into Kopaka, Toa of Ice. Upon arriving at Ko-Koro, they are attacked by the Rahkshi who had already ransacked the village, and escape down the side of a mountain. Kopaka is knocked out in the escape, and Takua tries to lure the Rahkshi away across a lake. Just as the Rahkshi close in, Kopaka awakens and freezes the Rahkshi in the lake. Kopaka then leaves to see to his village and the Matoran leave for the underground region of Onu-Wahi.
Takua gets lost in the Onu-Koro Highway, an abandoned network of tunnels connecting Ko-Wahi and Onu-Wahi. He is surprised by Makuta, who appeared before him in the form of two glowing red eyes, who warns Takua that if he does not give him the mask, Jaller will die. Takua refuses, but panics and leaves Jaller and the mask behind. He arrives in the village of Onu-Koro in time to find Pohatu and Onua, the Toa of Earth, telling the village about the Seventh Toa. However, three more Rahkshi—with the power of fear (Turahk), anger (Kurahk), and hunger (Vorahk)—appear, sent by Makuta in response to Takua's refusal of his good will and quickly incapitate the two Toa. As the Matoran are forced to flee from their village, Takua feels that this was his fault as the Rahkshi are after him for being the true Herald. Tahu, Gali, and Lewa arrive to defend the village, but Tahu is hit by Kurahk’s anger energy, the poison from earlier and the anger completely infected him, causing him to attack Gali. He is stopped when Kopaka arrives and freezes him. Meanwhile, Pohatu and Onua cause a cave-in, and the other Toa are forced to flee the village. Gali, Kopaka, and Lewa later bind Tahu to a rock and free him from his madness.
Takua finds Jaller and after warning him about the second wave of Rahkshi and the fact that the Rahkshi were looking for the Herald, the two head for the Kini Nui, where the mask says the Seventh Toa would be found. However, the six Rahkshi caught up to them. at which point Takua finally admits he is the Herald. The Toa arrive and, united, defeat all the Rahkshi except Turahk. Turahk then tries to kill Takua, only for Jaller to take the blow instead. As the dying Jaller tells Takua how he was "always different", the Chronicler realizes that he wasn't the Herald, but rather the Seventh Toa himself. Takua puts on the Mask of Light and becomes the Toa of Light, renaming himself Takanuva. He destroys Turahk, and then builds a vehicle out of the Rahkshi’s armor, called the Ussanui - powered by the discarded Kraata of the Rahkshi - and flies down to Makuta's lair.
There, Makuta challenges Takanuva to a game of Kohlii (played using a ball made of a substance called protodermis) with the agreement that the Toa of Light would try to awaken Mata Nui if he won or relinquish his mask if he lost. Takanuva prevails, using his special Kolhii move from earlier to knock the Makuta clean through a wall. Makuta, however, rises again and claims he will protect Mata Nui from the Toa of Light - stating that "sleep spares him pain...awake, he suffers" (possibly foreshadowing the end of the 2008 storyline, in which he took over Mata Nui's body and cast him out of the universe) and this duty is to his Mask of Shadows. When Takanuva tries to rip the mask off of Makuta's face, they fall into a pool of energized protodermis and emerge as Takutanuva. The combined being leads the Matoran, Turaga, and Toa (who had been led down to witness Makuta's defeat by Hahli, a Ga-Matoran and close friend of Jaller), through a gate in the Kanohi mask statue. Takutanuva, who is holding the gate up, resurrects Jaller by transferring part of his life energy into Jaller’s mask. However, this transfer weakens Takutanuva and the door crushes him, with only the Mask of Light remaining. They then proceed to attempt to awaken Mata Nui, reviving Takanuva in the process, and his light makes a hole in the cavern, revealing the long lost city of Metru Nui.