It seemed that you finally escaped Durandal's grasp at the end of Marathon 2, yet for reasons unknown the rampant AI has returned in its stolen alien vessel with a distressing message: a terrible force has been unleashed in the galaxy and the very fabric of time and space is at stake. As the primary enforcer of Durandal's will you once again take on the role of the lone security officer from the Marathon, your quest this time being to follow Durandal's conscience as it warps from different locations in time and space and somehow make sense of what is going on.
Marathon: Infinity is the third and final entry in Bungie's FPS series. It uses the same engine and graphics from its predecessor but features a whole new slew of maps to fight through as well as including a branching storyline that takes the plot to different places depending on how you complete each level. A new feature allows physics models to be built into maps, meaning parameters controlling everything from the behavior of weapons to gravity to which monsters are friends and which are foes can change from level to level.
To further increase value, the game includes Forge and Anvil, the level and object editing tools used by Bungie to create the Marathon games as a bonus feature.