Following the events of the original game, the Pfhor aliens have been defeated and the A.I. known as Durandral has been freed from it's containment. 17 years have passed and Durandral has assembled his own army from the colonists of Tau Ceti, including the Security Officer. Traveling across the galaxy, Durandral finds the home world of the S'pht, a Pfhor slave race, who Durandal liberates and who now worship Durandral as their Messiah. Durandal is searching for the location of a legendary lost tribe of S'pht who may hold the key to his mad quest for... well, whatever he's madly questing for. Unfortunately, the Pfhor dislike Durandral's presence and begin attacking his army.
Marathon 2: Durandal is the sequel to the first Marathon. Although the engine is similar to Doom in most respects (2.5D maps, bitmap sprites instead of polygons, etc.), Marathon 2 distinguishes itself with such features such as: a rudimentary inventory system, ambient sound, multiple fire modes for each weapon, allies who aid the player, enemies belonging to different factions which will attack each other, a more developed plot (gradually narrated to the player at various computer terminals scattered throughout the levels (much like System Shock's e-mail transmissions)).
The game's engine has gained a number of improvements over the original Marathon. It now displays in a much larger portion of the screen, supports maps with four times the complexity and adds support for liquids and swimming. All liquids use oxygen when the player is submerged and some also do damage. Many levels have mechanisms allowing areas to be flooded or drained, changing the depth of the liquid.
The other major new feature is a bunch of new multiplayer modes. The entire single-player portion can be played cooperatively with up to eight players and the game adjusts weapon and monster placement accordingly. In addition to "Every Man for Himself" mode (essentially deathmatch), it now includes "Kill of the Hill" where the object is to occupy a designated spot on the map more than anyone else, and "Kill the Guy with the Ball" where the object is to hold a skull the longest, the player holding it being unable to fire weapons.