Bungie's rather under-recognized precursor to the famous Halo-series, which takes heavily from it's older sibling. You begin as a random security officer, answering a call for help from the colony ship, Marathon. But there seems to be far more going on then it seems, the ship has been invaded by an alien empire called the Pfhor, who seek to enslave humanity. The games go far more in-depth after that, which for it's time was an innovation for gaming as it was one of the first (if not the first) FPS game to have an in-depth story in game.
The story and your objectives are told to you through terminals you find throughout the game. Manning them are usually on of the A Is
(or Robert Blake) who perform the send you on your way. You have Leela, the only "good" A.I, in your encounter trying the use you to save humans on the Marathon. Durandal and Tycho are a different story, both have gone rampant and are extremely cunning to boot, Durandal in particular can come off as a wild card, with many interpretations of him depending on the player. Other characters appear on these too, such as the ever-enigmatic Thoth, the mysterious AI (with no stated name), and Robert Blake usually.
Gameplay wise the game was also an innovator of it's time. The graphics were (for it's time) very good for starters. When it came to shooting, it was the first game to allow you to aim up and down, dual-wield, and have clips (though there is no reload button). There was something of a jump, but it was more of a glide and one had to be careful in moments where they traverse obstacles this way as you can "bounce" strait into the hazard if not careful.
The story itself is very in-depth and thought provoking. With many things up for interpretation or strait up mindscrewing most players. There is even an entire website devoted to WM Ging
regarding this game. Especially the third game (Infinity), which is Bungie's version of the C'thulhu mythos apparently.
Brilliant games, and they are now freeware for anyone to download if anyone wants to try them.