Marathon Infinity was the third and final game in Bungie's Marathon trilogy of first-person shooter games. The title is a dual reference: first, the time-looping nature of the game's story, and second, the included game development tools.

Marathon Infinity uses the same gaming engine as its predecessor, Marathon 2: Durandal, so to make up the difference Bungie included their map-making tool (Forge) and their shape/sound/weapon creation tool (Anvil) on the CD-ROM along with the game. Similar tools had been created by ambitious fans before this time, but Bungie's "official" tools made it possible for anyone and everyone to extend the Marathon game "infinitely". Several new campaigns based on the Marathon universe have already been produced, and fans are still doing so. (These campaigns will be, or should be, compatible with the open-source Marathon: Aleph One engine currently under development by fans.)

The story for this game is very confusing, compared to the ones for Marathon and Marathon 2: Durandal, and the manual for the game is needed to help provide some insight. At the end of the second game, the Pfhor have been routed by Durandal and have deployed a weapon intended only as a last resort, which causes the sun of the S'pht homeworld Lh'owon to go nova. But back when Lh'owon was first colonized, a race of immortal and chaotic creatures named W'rkncacnter were imprisoned in that star. As it goes nova, the W'rkncacnter are released, unleashing chaos and destabilizing reality as they emerge.

The result is that you, the durable hero of the Marathon series, are cast back in time and placed under the manipulations of Tycho, Durandal's adversary, shortly before Marathon 2: Durandal begins. Throughout the game you're bounced around between Tycho, Durandal, the Pfhor, and the strange S'pht AI whom Durandal dubbed Thoth, forced to obey the desires of whomever's in charge at the moment. Your ultimate goal is to prevent the W'rkncacnter from escaping the early nova, and in the meantime you gain some very interesting insight as to what happened during Marathon 2: Durandal that Durandal didn't or couldn't tell you.

Like the other Marathon games, the story unfolds in a series of computer terminals placed throughout the levels; however, it's rather difficult to read it outside of the context of the game. A large part of the game is deliberately nonlinear, and you can skip entire levels or loop back through old ones if you locate certain terminals. Second, much of it reads as oblique metaphor, which looks like nothing more than obscure nonsense until you've played the game through about three times.