999: Nine Hours, Nine Persons, Nine Doors is a visual novel mystery/puzzle adventure game, and the first entry in the Zero Escape series.
In the game you play as Junpei, one of nine random people who are mostly complete strangers to one another. You and the others wake up to find themselves trapped on an abandoned cruise ship, only to then be informed by a mysterious figure that they have 9 hours to escape the boat together before they are all killed. You travel through the ship and its wide assortment of bleak, lonely, and sometimes unnerving locations in a first-person perspective, while talking to 2D characters with excellent sprite work, solving puzzles, and exploring 3D-rendered environments in a style somewhat similar to Myst.
The strongest points of 999 are its excellent atmosphere and writing. Every empty room that resembles a normal room, yet stripped of functionality or any view of the outside world, cements the feeling that you are trapped in hostile location against your will. This is all backed up by the excellent soundtrack and sound design, as the often-eerie and droning music is accompanied by the heavy creaking of the ship to create an oppressive tension in the air, making you feel as though you are trapped in the belly of a massive, unfeeling beast.
The writing, again, is phenomenal. Every character is distinct and memorable, and each of them carries various different motivations for their actions -- both obvious and hidden. As you make your way towards the unknown and unseen exit, you must work together with the other people in order to succeed. Yet at the same time, you have no reason to trust them, and some of them give you perfectly legitimate reasons to distrust them entirely. Death and plot twists are around every corner, and events take an unexpected turn early on, which ramps the tension up high right off the bat.
It must be clarified that despite all of the game's positive qualities, 999 is designed in a way that expects the player to play through the game several times in different ways in order to eventually discover the true ending. There are multiple bad endings, and dying about halfway through and starting all the way over is a very real possibility. Combine that with the large amounts of text you are expected to read, seeing as this is a visual novel, and you can see that this game isn't for everyone.
But if it looks interesting to you at all, and you're looking for well-written, dark, and truly gripping story, I'd highly recommend you look further into 999.