Back in its original release in December of 1994, Marathon (Classic) would be called a Doom clone...However, this (and its original release) FPS is anything but. It was originally released for the Macintosh but later ported to its current game engine called AlephOne.

Plot primer:
You are Security Officer Mjölnir Recon #54 and have been awakened from stasis by the colony ship UESC Marathon's AI named Leela. All hell is breaking loose on board the Marathon while an unknown race of aliens are attempting to take over the ship to find information about its origins so it can enslave the human race. You and the AI onboard are the ONLY resistance the aliens meet as you go about the ship surviving each encounter, doing what the AI tells you, and trying to save Humanity as we know it.

Game Play:
Marathon was one of the first FPS's to have vertical aiming with a mouse, and an immersive story line. It is recommended you carefully read the terminals you come across to understand the lore of the Marathon universe...and in most cases, what to do to get to the next jump pad.