At the start of the game, the player character is aboard a shuttle returning from the colony to Marathon when an alien ship attacks the system. The officer makes his way to Marathon to find that the aliens used an electromagnetic pulse to disable much of the ship. Of the three AIs, only Leela is functional, and she guides the officer in a counter-strike against the aliens and to restore the other AIs and key systems. Leela learns that Durandal (one of the shipboard AIs) had been in contact with the aliens prior to their engagement with Marathon. The alien race, known as the S'pht, are being forced to fight by the Pfhor, an insectoid-like race. Leela soon discovers that Durandal had become "rampant" before the attack, and is able to think freely for himself. Leela aids the officer to disable Durandal's access to vital Marathon systems while sending a warning message to Earth, but in turn Durandal has the Pfhor send more forces to attack the Marathon, ultimately kidnapping the security officer. Leela intercedes to free the officer, but warns him that the S'pht attack has nearly destroyed her systems. The officer races to complete a bomb in the ship's engineering rooms, hoping it will force the Pfhor and S'pht to leave, but it is too late as Leela is "killed" by the S'pht, and Durandal takes over, forcing the officer to continue to follow his orders to stay alive.
Durandal has the officer repair the ship's transporters, allowing him to go aboard the alien Pfhor vessel. Inside, while fighting off the Pfhor, the officer discovers a large cybernetic organism that the Pfhor use to control the S'pht. The officer destroys the organism, and guided by Durandal, the S'pht revolt against the Pfhor, first on their ship, and then aboard the Marathon. With most of the Pfhor threat gone, Durandal announces his intention to transfer himself to the Pfhor ship, which the S'pht have control of, and leave with them. As a parting gift, Durandal reveals that Leela was never fully destroyed, and the S'pht release their grasp on her before departing. As the alien ship departs the system, the officer works with Leela to clear the last remaining Pfhor aboard Marathon before assessing the full damage that has been done.
Gameplay
Gameplay takes place in a real-time, 3D rendered world with ceilings and floors of various heights and widths. All surfaces in the game are texture mapped and have dynamic lighting. The player controls the movement of the main character primarily through use of the keyboard. Using assignable keys, the player can move forward and backward, turn left or right, sidestep left or right, look up, down or forward, and glance left or right. Additionally, Marathon features free look, where the player uses the mouse to rotate character view. The mouse may also be employed to fire weapons. Marathon was one of the earliest games to employ free look and give the player the ability to look up or down.
The player progresses through the levels in sequence, killing enemy creatures and avoiding numerous obstacles while trying to survive. While the player completes the levels in a fixed order, many levels are non-linear and require extensive exploration. Obstacles include "crushers" (ceilings that crush the player), pits of harmful molten material or coolant, locked doors or platforms that must be activated by remote switches and puzzles that may involve precise timing and speed to complete successfully. Some levels have low-gravity, oxygen free environment and/or magnetic fields that interfere with the player's motion sensor.
The plot is fundamental to gameplay and player advancement. The primary channel through which this plot develops is the computer terminals located throughout the game's levels. The player accesses these terminals to interface with the artificial intelligences of the Marathon, who also provide him with information regarding the levels.
Multiplayer
As many as up to eight players (each playing through a separate computer) on a single computer network can participate in a single match, either in teams or every man for himself. A player or team's score is their number of kills minus their number of deaths. The game may be set up to end after a specified number of minutes or total number of kills. After a match concludes, the results of a game are displayed in graph form upon each player's screen.
Marathon includes ten multiplayer maps. When a player dies on one of these maps, he will respawn randomly at a respawn point, if enabled by the gatherer, penalties for suicides or dying will apply and prevent a player from respawning immediately.
Marathon's multiplayer was one of its most anticipated features prior to release and won Marathon the Macworld Game Hall of Fame Award for the best network game of 1995. Bungie reportedly intended to add more multiplayer scenarios such as cooperative play but could not due to time constraints. Many of the concepts and levels that could not be included in the final product because of a lack of time to implement them were included in Marathon 2. Bungie has reported that the development of Marathon was delayed significantly due to time spent playing the deathmatch. The code was written almost entirely by Alain Roy who reportedly received a Quadra 660AV in compensation for his efforts. According to Jason Jones, the network code is packet-based and uses the DDP, or Datagram Delivery Protocol to transfer information between each machine.
