A thrilling visual novel that tells a story that could only be told as a video game.
The opening scene is set w/ the main character, Junpei, waking up and finding himself in a cabin on a ship. Almost immediately, a porthole explodes rapidly filling the cabin w/ water. In order to escape, Junpei must solve a convoluted environmental puzzle to save his very life. The immediacy of this opening gives way to a twisted and over-the-top tale of malice, far-out science and deep personal connections.
So, yeah, it sounds pretty much like a bunch of overdramatic garbage on paper. The amazing thing is though, it works. The story told here would a bad mishmash of Primer and the Saw movies had it been presented as a movie. But it's a game. And it works as a game.
The basic idea here is Junpei can go through the choices he makes again and again w/ the knowledge the gamer has based on previous playthrough. In most games, gamers just accept that this for what it's worth: even though you've played through a few times, there is really only one relevant timeline, which is the timeline currently being played. 999 turns this notion on its head. It really matters that the game has been played through before.
In fact, the game can't even be beat on the first playthrough--if you even can use such a term for a game where your primary reason to keep playing is to discover more plot details. You have to play through at least twice to get the full ending, which most people would only be able to do using a walkthourgh since some seemingly inconsequential decisions have a big impact on the final outcome, and many times more than that if you want every last detail 999 has to offer.
Ultimately, it is simply just amazing to unravel the incredible ball of yarn that 999's creators have spun. The puzzles are good fun and some are quite challenging but you solve them not for the sake of it but because the creators give you a reason to want to move on. It is not just to save Junpei's life or those of the other characters, it's because you yourself want to know the same things Junpei does. As much as that, it is simply a marvel how well such a story fits in a game and one of the few tales I know of that could only be spun that way.