MegaTraveller 2: Quest for the Ancients

300,000 years ago, the ancients ruled the galaxy. It's now ~4000 AD and they're long gone. Unfortunately, one of their pyramids has started spewing slime that threatens to engulf the entire planet of Rhylanor. The player's party of adventurers must scour four subsectors of the Spinward Marches looking for clues to solve the crisis before Rhylanor is destroyed.

MT2 is a very faithful adaptation of the Traveller role-playing game with over 100 fully explorable planets (most with multiple cities and each with it's own sub-quests) and a non-linear story line.