The third title of the three games collectively known as the Marathon Trilogy, you reprise your role as a security officer, you must rewrite time to stop an eldritch being named the W'rkncacnter, which was released from its imprisonment in Lh'owon's sun, this beast of chaos, if left unchecked, would spell the death of the known universe!
The Marathon Trilogy was quite a head of it's time when it was released, taking the Wolfenstein 3D and Doom formula and adding a proper story/narrative to it for the first time, as well as introducing things like vertical aiming, health and shield recharge stations, dual-wielded weapons (as well as the need to reload them), proper multiplayer, physics, mission objectives that require more than just pressing the exit switch at the end of each level whilst mowing down hordes of enemies, and proper modding/editing tools.
It's such shame not many people know about this series of amazing games, considering they're freeware and made by Bungie, who would later make the Halo and Destiny games.
I think every FPS gamer should download and play every Marathon game to end! :3