From the back of the book:
Infinite Worlds . . . Infinite Adventure!
The good news is, there are other Earths. Maybe an infinite number. The bad news is, somebody out there doesn't like us.
The shuttles of Infinity Unlimited jump between parallel Earths, seeking adventure, profit, knowledge, and even entertainment. But a parallel called Centrum has also developed the technology to hop between the worlds . . . and they want to rule them all. The Infinity Patrol must deal with their ruthless rivals, as well as with world-jumping criminals, and with the possibility that the secret of dimension travel might escape to some of the really nasty alternate worlds like Reich-5.
Welcome to the core setting of GURPS Fourth Edition! Every other GURPS setting is on one of the Infinite Worlds timelines . . . whether they know it or not! GMs can use this to create a whole meta-campaign, or just as an excuse to move characters between worlds when the plot requires it.
Compiled by Kenneth Hite, the master of alternate histories, GURPS Infinite Worlds combines and updates material from Time Travel, GURPS Alternate Earths, and GURPS Alternate Earths 2 into one full-color hardcover volume, and gives dozens of new worlds to explore as well! This is the complete genre book on both alternative-world gaming and time travel. It offers detailed advice on the unique challenges of running this kind of campaign, and on designing and playing characters who regularly cross between settings. It also provides a wide variety of suitable threats and hazards - from evil cross-time Nazis and cosmic conspiracies to "ordinary" monsters and disasters. And it gives guidelines for building alternate worlds from the perspectives of the setting, the story, and the rules.
Whether you're playing accidental travelers or the hardened troops of the Infinity Patrol, this book is your gateway to adventure. Infinite adventure.