GURPS: Infinity Patrol
The shuttles of Infinity Unlimited jump between parallel Earths, seeking profit, knowledge, and adventure. But a parallel called Centrum also has the technology to cross between worlds… and they want to rule them all. The Infinity Patrol must deal with these ruthless rivals, as well as with world-jumping criminals, and try to keep the secret of dimension travel out of the hands of the really nasty alternate worlds like Reich-5.
Welcome to the core setting of GURPS Fourth Edition! Any campaign can be on one of the Infinite Worlds timelines… whether they know it or not!