You are on a spaceship that has been invaded by aliens, and it is your job to save it. The plot reveals itself as you play. After you've had your several hundred hours of fun with the included scenario, there are hundreds of new levels and scenarios available off the Internet, many of which rival the original scenario that Marathon shipped with. Here are some screenshots. These two games boast improved graphics and new scenarios.
Infinity comes with a map editor, so there are thousands of levels and scenarios available for it.
The Marathon 2 and Marathon Infinity maps, shapes, and sounds can be used with a new open source graphics engine, Aleph One, for better graphics and new features. It's called Marathon 2. After we installed it, he was enthralled in the gameplay. He was even more impressed after we played each other in a netgame. The Marathon Series, in my opinion, is the best series of games ever released.