Marathon Infinity included more levels than Marathon 2, which were larger, scarier, and part of a more intricate plot. The game's code changed little since Marathon 2, and many levels can be played unmodified in both games. Marathon Infinity was only released for the mac Macintosh. The most dramatic improvement in the game was the inclusion of Bungie's own level-creating software, Forge, and their physics editor, Anvil. Forge and Anvil allowed a new generation of players to create their own levels using the same tools as the Bungie developers themselves. In Forge, distance was measured in World Units, which are roughly equivalent to 2 metres (6 or 7 feet). Another improvement was the ability to include separate monster, weapons, and physics definitions for each level, a feature heavily used by Double Aught, who designed the Marathon Infinity levels
Marathon Infinity begins as the Pfhor destroy Lh'owon using a stolen Jjaro doomsday weapon known as the Trih'Xeem (early nova). Unfortunately, the weapon also releases a powerful chaotic being which threatens to destroy the entire galaxy. Because of the chaos, or by means of some Jjaro tech of his own, the Marathon Cyborg is transported back in time and finds himself jumping between timelines and fighting for various sides in a desperate attempt to prevent the chaotic being's release.