Marathon Infinity

Core game mechanics change little from Marathon 2 except for the addition of a new weapon, the "KKV-7 10mm SMG Flechette". The player is placed into a usually semi-nonlinear level and is generally given a task which must be completed. Upon completion of this objective, the player then proceeds to an extraction point, usually in the form of a terminal. In Infinity, there are a few major deviations.

Unlike previous games, certain actions will cause the game to branch out. The game does not contain multiple endings, as these branches will eventually merge back into the main story. The game also makes much greater use of plugin physics models that change game settings from level to level. This is most evident from the player's constantly changing allies throughout the game, as almost every creature in the game will act as both allies and enemies as the game progresses.

The story in the single-player version of Marathon Infinity, titled "Blood Tides of Lh’owon", is not told in an explicit fashion. The narrative begins as if large parts, if not all, of the events in Marathon 2 had not happened. At the end of Marathon 2 proper, as the Pfhor's Trih Xeem or "early nova" device is fired upon the S'pht System's sun to explode it, Durandal recounts an ancient S'pht legend in which a chaotic entity known as the W’rkncacnter — an eldritch abomination — was sealed inside of that sun by the Jjaro — a highly advanced race from centuries past, their technology being the only remnants of their existence — eons ago. The story involves the player "jumping" between alternative realities via surreal dream sequences, seeking to prevent the W’rkncacnter from being released from Lh'owon's dying sun.

These jumps are apparently caused either by technology left behind by the Jjaro or by the W’rkncacnter's chaotic nature. The player begins as Durandal's ally, only to be transported almost immediately to a reality where Durandal did not rescue the player at the end of the first game, Marathon; as such, he is controlled by the Pfhor-tortured AI Tycho instead.

Allgame editor Alexander Goldman described Marathon Infinity as "the standard against which other Macintosh shooters are compared".

The editors of CNET Gamecenter named Marathon Infinity the best Macintosh game of 1996 and wrote that its "design and playability [...] pushed it into the stratosphere".