Marathon: Rubicon is a total conversion set fifty years after the events of Marathon Infinity. Rubicon is one the largest scenarios ever created and upon the release of Aleph One, a new version was created to use the enhanced capabilities of the engine, called Rubicon X. It contains 83 levels,
The story is kind of based on your choices and actions, though this is only at the end of a so-called plank. A plank is a major part of the story, split into chapters, containing levels (obviously). The action you take at the end of the first plank, the Chimera Plank, will lead you to either the Salinger Plank or Pfhor Plank, each having a different plot. And at some point it is possible to go to the Tycho Plank.
You start the game in a swamp-like area, which is later revealed to just be a dream or vision. These levels are called Dream Levels, because it usually does not make much sense. Then you wake up on the Rozinante, Durandal's ship, where you get your first objectives. There are eleven Rozinante levels in total.