…for the original doom, the overarching narrative was that deimos disappeared after teleportation research inadvertently opened a portal to hell: episode 1 (knee-deep in the dead) was set on phobos; episode 2 (on the shores of hell) was set on deimos hovering in the skies of hell; and episode 3 (inferno) was set down on the surface of hell itself, not mars…
…episode 4 (thy flesh consumed) was set back on earth following the invasion of hell, but it was an add-on released a couple years after the original game; counterintuitively, back then the original game was downloadable shareware for episode 1 with episodes 2-3 available via mail-order directly from id, and episode 4 was finally distributed as a retail expansion only after the retail release of doom II (kind of an inter-quel?)…
…in `94, i mailed a handwritten check to id software and received a slim box of 3.5" diskettes a couple weeks later; the world was a different place…