I just finished playing Bionicle: Masks of Power 1.5 Legacy, a feat which took me five days of playing, off and on, though having played the earlier betas helped me solve some of the puzzles faster. And the developers clearly put a lot of work into making a full-scale fully traversable replica of the island and its denizens as an Unreal Engine 4 game with Skyrim-style open-world exploration. There is no other such comparable experience as of yet, so props and credit where it is due.
The basic premise is this: you play as the six Toa, going around the island of Mata Nui, collecting Kanohi masks and Makoki stones, fighting Rahi, completing quests, and finally coming together to fight the evil Makuta. More or less the familiar '01 story experience.
But here's where things get a little odd: Many of the NPCs you talk to address you as if they've met you before, but you reply as if this is your/their first meeting. And the Turaga are clearly keeping secrets, even more so than those of you familiar with the Metru Nui Saga would know they are. Most of the Turaga become trapped and in need of rescue after returning from a secret meeting, and you must go on quests to retrieve them. Takua even gets trapped in Ta-Kini, and after Turaga Vakama sends Toa Tahu to rescue him, Takua tells the Toa that Vakama sent him there to search for the Kanohi Vahi, the Legendary Mask of Time.
The Kanohi Vahi was cut from the first year of Bionicle, because the PC game it was supposed to be packaged with, which is not unlike this one besides not being open world, got cancelled right after 9/11 and was only recently released in the last few years. The Vahi then became a special promotional item only available from the Walgreens mail-in catalog in June 2002. Story-wise, the Vahi was in Vakama's possession during the '01 saga and didn't appear until the middle of the '03 saga, when he gave it to Tahu to fight the Kal. But I digress.
The point is, all these confusing hints and details factor little into the gameplay and plot until you finally collect all six Golden Masks and gather at Kini-Nui for the final battle. Then, just as the Toa are about to descend into the Makuta's lair, the six Turaga show up out of nowhere and confess to them a terrible truth. Much as anyone who had the displeasure of playing E.Y.E: Divine Cybermancy found out about the vague hints leading up to its barely coherent non-ending, the Turaga inform the Toa that the reason they can't remember their prior meetings with the Matoran is because the Makuta has them trapped in a time loop.
Apparently, the Makuta has somehow gotten his hands on the Kanohi Vahi, and has been using it every time the Toa confront him to reset their memories, belongings, and positions to the scattered states they were previously at when they first arrived on the island. They apparently fought the Makuta many times, only to wake up tabula rasa and start all over again each time. The Turaga tell them this in the hopes that that knowledge will enable them to finally break the cycle.
You descend into Mangaia, fighting the Makuta's many fearsome Rahi and your evil doppelgängers the Shadow Toa, before finally confronting the Makuta himself. The final battle begins as the Mata Nui Online Game's did, but then the dialog takes a turn when the Toa mention they're putting a stop to the time loop. The Makuta says that they're right, because while he had fun defeating and resetting them before, now that the Turaga want them to know the truth, it's time for him to kill them all.
They combine their powers, just as they did in MNOG, defeating the Makuta. But he's a sore loser and tries to collapse his lair on them. Tahu then takes the Vahi from him and uses it to save them all by resetting time again, but in doing so, finally breaks them from the cycle. He hopes they can prevail against the Makuta again the next time. The game ends with an assurance that it will continue in the game's upcoming remake, Bionicle: Masks of Power 2.0 Rebuilt, already in development.
If you load your latest save after that, you'll be right back where you started at Tahu's Landing on Ta-Wahi Beach. You'll still have all your remaining gear and Kanohi masks, and you can still explore the island and fight Rahi, but you will have no further main story quests. And as a completion bonus, the next time you go to a cosmetics bench, you'll see that you have an additional wearable item: the Kanohi Vahi, positioned on top of your currently worn mask, with flavor text indicating that it's a trophy taken from the Makuta in his lair.
I suppose the implication of all of this is that this game was a prequel to the upcoming Rebuilt version of the game, and Legacy is the story of what happened just before it. Though I can't be sure about that until it actually comes out.
So, if you're looking for a chance to play a 3D recreation of the island of Mata Nui and the classic '01 Bionicle story saga, you should check out these four choices:
Lego Bionicle: The Legend of Mata Nui Rebuilt (a fan-completed version of the aforementioned canceled game, which I've also finished playing, but its fan dev team is working on additional major updates)
Bionicle: Masks of Power 1.5 Legacy (this game, which is even less canon than The Legend of Mata Nui but generally a better gameplay experience, though it will not receive any major future updates)
Bionicle: Masks of Power 2.0 Rebuilt (the remake of this game, currently unreleased and in development)
Bionicle: Quest for Mata Nui (a similar Bionicle '01 recreation project using the Starcraft 2 Engine, currently unreleased and in development)