From the back of the book:
The world we know is one of many planes of existence. Some are alternate worlds, where history took strange, even apocalyptic paths. Some are Otherworlds: weird realms where magic leaves its blazing mark through cities built on the back of monsters, and the floating warships of demigods. Finally, demon princes with wills of iron rule the Netherworlds, planes with twisted natural laws and legions of tormented souls. In ornate archways or simple alley doors, gates connect the myriad planes. You know where to find them. You’ve been initiated into the truth.
You might belong to the Sodality, an organization that protects the innocent and explores the planes, or you might be an agent of Aethon: a transhuman operative who protects Earth from strange forces, and manipulates the histories of alternate worlds for your AI masters. Battle gods and cyborgs, navigate the cold war between transplanar empires, and uncover the mysteries at the heart of Threefold.
The first original setting for the Modern AGE roleplaying game, Threefold presents a vast Metacosm able to support virtually any character concept—even any genre—within it. Threefold includes characters with extraordinary powers, and provides additional rules and powers for soul talents, magic, psychic disciplines, occult uals, and technological enhancements.