BIONICLE 2: Revenge of Karzahni is one of the 2 sequels to BIONICLE: THe movie.
It is revealed that thousands of years before Mata Nui and the other BIONICLE arrived on Planet LEGO, a race of ancient BIONICLE scoured the universe looking for Energy sources. Known as the Seven Nuis, they used a device called the "Solar Harvester" to drain stars, such as the Sun, of their energy in order to convert it to Energon and power Metru Nui’s Mask of Life and their life-blood. The Nuis agreed that life-bearing worlds would be spared, but one brother, thereafter dubbed "Karzahni", constructed a Solar Harvester on Planet Lego in 17,000 BC. The remaining brothers sacrificed their bodies in order to hide the Maskof Leadership, the key that powers the Solar Harvester, from Karzahni, who swore to seek revenge upon Planet LEGO once he found the key.
In the present day in Sakoshima, there was a "major toxic spill", and M.I.K.E was evacuating the people. Mata Nui now leads MIKE, a military organization consisting of minifigure troops and his own team of Toa - including newcomers Parkani, Raknia, Tripas and the Fire twins Bob and Steve. While on a mission in Sakoshima to kill Makuta Vezon (who is killed by Tripas) and Zaktan (shot in the face by Mata Nui), they learn that "Karzahni shall rise again." Back in the Brick States, Joe Brickenson finds a splinter of the destroyed Mask of Life, and upon contact the splinter fills his mind with Metru Nuian symbols, which he is initially unaware contain a riddle that leads to location of the Mask of Leadership. Joe gives the Mask of Life splinter to his girlfriend Susan Brickla for safekeeping, and leaves her and Tahu behind to go off to college. Upon arrival, Joe meets his college roommate Leo LEGOz, who runs an extraterrestrial conspiracy website, and Mary, a co-ed who makes sexual advances towards him. Back home, Toa-turned-Makuta Jaller attempts to steal the shard but is captured by Susan. She interrogates him with a welding torch, and immediately leaves to get to Joe, as he has had a mental breakdown, writing uncontrollably in a Metru Nuian script.
Makuta Haakan hacks into a BS satellite to get information on the MIKE forces, learning the locations of the dead Makuta leader Teridax and another existing piece of the Mask of Life. The Makuta retrieve the shard and use it to resurrect Teridax with the help of Nuiacka and Scapera (who kill one of their own for parts, as none were available). Teridax flies into space and reunites with Spiriah and his master, Karzahni. Karzahni instructs Teridax and Spiriah to capture Joe in order to discover the location of the Mask of Leadership as well as to destroy Mata Nui, the only possible hindrance to his plans.
Susan arrives in town just as Mary, who is revealed to be a Pretender, attacks Joe. Susan, Joe, and his roommate Leo destroy Mary, but are captured by Kraata. The Makuta Vultraz (known as "The Doctor") prepares to remove Joe's brain, but Mata Nui crashes through the ceiling while Tahu bursts through the wall. A short firefight ensues as Tahu fends off Spiriah, allowing the humans to escape, while Mata Nui lands, pulling out his second sword, preparing to take on Teridax. Mata Nui takes on both Spiriah and Teridax in a barrage of shots and as Spiriah is sent reeling from a few close shots, Mata Nui fires a single round which sends Teridax through a wall. Mata Nui collects Joe while Teridax flees and they head towards a forest as Tahu evacuates the rest of the minifigures. Teridax quickly catches up, engaging Mata Nui in close-quarters combat while exchanging insults and punches. Makuta reinforcements quickly arrive and a fight ensues among Mata Nui, Teridax, Spiriah and Kraata. During the fight, Spiriah loses his right arm and Kraata is killed. However, Mata Nui ends up getting killed by Teridax while distracted.
After Mata Nui's death, Teridax orders a full-scale assault on the planet. Karzahni speaks to the world by transmitting a television transmission from a satellite hacked by Hakaan and demands that they surrender Joe to the Makuta or the attack will continue. Joe, Susan, Leo, Tahu, the Fire Twins and Jaller regroup, with Leo believing that his online rival "LEGOWarrior" may be of assistance. "LEGOWarrior" is revealed to be former Sector 8 agent Seymour Brickens, who informs the group that the Bionicle have visited Planet Lego before, as their language is written on ruins all over the world. Wheelie identifies the language as that of the Nuis, and directs the group to a former Makuta named Miserix, who can translate the language. They find and reactivate Miserix at the Bricksonian. After Miserix goes on a rampage through the aircraft museum, Joe tells him about the symbols he keeps seeing in his head. Miserix understands them and teleports the group to LEGO Egypt where Joe's hand gets injured during the ejection. Miserix explains the history of Karzahni and that the tomb of the Nuis is located in the surrounding desert, and only a Nui can kill Karzahni. Miserix provides them with a vital ancient clue: "When the dawn alights the Dagger's Tip, three kings will reveal the doorway." The group locate the Pyramids of LEGO, referred to as the Dagger's Tip, and spend the night until dawn. When the sun rises, Joe recognizes three outstanding stars near the horizon, having gone through the entire astronomy textbook earlier, as being called the Three Kings. They follow the stars and locate the tomb of the Nuis, where a brawl between Bob and Steve breaks a wall and reveals the entrance to the tomb. The Mask of Leadership is located inside, but crumbles to dust in Joe's hands.
Believing that the Mask of Leadership can still revive Mata Nui, Joe collects the dust in his sock and instructs Brickens to telephone Major John Hicks to bring the other Toas and Mata Nui's body to their location. The military arrives with the Toa, but so do the Makuta, and a battle arises. During the fight, the largest Makuta called Kardas is formed and unearths the Solar Harvester, concealed within one of LEGO Egypt's pyramids before being destroyed by the BS military's prototype rail gun. Miserix arrives in time to save the group from Icarax by slicing him in half and smashing his head off, but is mortally wounded by Mutran, whom he killed. The Air Force carpet-bombs the Makutas, but Teridax breaks through the offensive and attacks Joe. In a vision, Joe meets with the other Nuis, who tell him that the Mask of Leadership is not found but earned, which Sam has done, and they send him back.
The dust reassembles to form the Mask of Leadership, which Sam uses to revive Mata Nui. However, Karzahni jumps into the fray and steals the Matrix shortly thereafter, rendering Mata Nui unable to fight once again. Karzahni activates the Solar Harvester. In a final effort, MIserix sacrifices his parts and spark to Mata Nui, giving Mata Nui the power he needs to defeat Karzahni. With his new, enhanced capabilities, Mata Nui destroys the Solar Harvester by shooting it down and takes on both Teridax and Karzahni. After severely damaging Teridax, Mata Nui faces off against Karzahni and kills him by tearing away part of his faceplate then forcing his fist through his torso and crushing Karzahni's spark with his bare hands. Teridax and Spiriah retreat and Teridax vows that their fight is not finished.
Mata Nui concludes the event by proclaiming that the BIONICLE and minifigures now share an untold history and will combine despite their differences.