From the introduction:
A long time ago, with advanced technology or maybe magic, an Interdimensional Tunnel was built on the PC’s homeworld. Meanwhile, across countless other realities, the engineers’ counterparts were building parallel tunnels of their own. When activated, all of these alternate tunnels fused together, forming an infinite hallway that bridged endless realities. Realizing there were numerous new worlds to explore (or exploit), the inhabitants of most of these worlds left for greener pastures, leaving long stretches of the Hallway connected to mostly empty and isolationist locales. This adventure takes place in one such stretch of sparsely inhabited worlds, marked for doom by the prophecies of the mysterious Longers. Adventure through the Hallway to explore new worlds, reconnect dying civilizations, and discover the secret of The Doom.