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The good news is, we can visit other Earths.The bad news is, somebody out there doesn't like us.Time Travel combined with alternative world hopping. GURPS: Infinite Worlds is the default GURPS 4th edition setting, loosely inspired by H. Beam Piper's "Paratime" novels. The successor to GURPS Alternate Earths and the Third Edition sourcebook GURPS Time Travel.The basic concept is this: A Parallel Earth (that calls itself "Homeline") develops the technology for travel to other timelines. It is controlled by the United Nations, which effectively becomes a World Government (an opinion not every nation and corporation agrees with). They secretly explore and exploit the other Earths (in relatively benign ways - buying resources from Earths that have plenty, for example, and dumping wastes on Earths that never developed life) and forms an organization known as "The Infinity Patrol" for the purpose of keeping "The Secret" (that is, the existence of The Multiverse) from those Worlds that do not know about it (the vast majority discovered so far).The reason is twofold. First, worldlines that learn The Secret may either be aggressive timelines, or become the victim of one, creating another reality-hopping danger. But the second deals with "echo" timelines that mirror Earth's past — changing an echo by interfering too much may cause that reality to "shift" closer or farther away from Homeline (that is, it becomes more or less easy to access.) In fact, Homeline's biggest rival, Centrum, a World that also has "cross-time" technology but with a Communist-type socialist technocratic society, seeks to draw worlds "closer" to themselves so they can influence them into developing similar governments. Occasionally, "outtimers" (slang for people from outside Homeline) are recruited to join Infinity if they are judged worthy. This is similar to a Time Police setting genre except it involves parallel worlds instead of the actual past or future. (The book also includes a separate, Time Travel setting called "Project Timepiece" as well as settings such as The Order of the Hourglass, a pulp setting where adventurers travel through time via their minds.)Oh, by the way there's a third up-and-coming player in the multiverse that recently extracted The Secret from a crashed Homeline conveyor. Homeline's name for their timeline? Reich-5.This setup allows GURPS game masters to permit Crossovers between different genres. For example, the Earth from GURPS Supers has been recently discovered, which could allow for crossovers between the two settings. This also gives both the player and GM a huge sandbox to play with. This is to some degree, the purpose of the setting, as it literally allows the game master and the players to goof around, err.. that is police the infinite multiverse where literally anything is possible (that includes some very weird possibilities) without worrying about actual time-travel (unless you want to of course) The sheer complexity and size of the multiverse and the parachronic physics is probably the main reason The Secret can be maintained with any success at all.
This, as a setting for GURPS, shares many tropes with GURPS.
Alien Space Bats: Centrum and Homeline do a LOT of this, as often as not as a part of their ongoing conflict. More worryingly, it seems that they may not be the only ones who do this, as the no less than 18 Gothas (parallels which all suffered a Zombie Apocalypse with a common source) seem to point to...
Arabian Nights Days: Caliph twists this. It's a world mostly covered by Islamic nations in the 1600s, except that the magic is technological...and the worldline is by far the most advanced of any that either Infinity or Centrum have ever discovered.
Reich-5 was originally presented as a sleeping giant, but while it's awakened, it suffered Diminishing Villain Threat between its initial presentation and the exposure of parachronics, and the latter event occurred in such a way that most of the Nazi hierarchy is unaware of the situation, so the Armanen and Raven Division are more of a Goldfish Poop Gang right now than a match for Homeline or Centrum.
Merlin's another partially-awakened giant: a world with technology equal to that of our own Earth, but with access to advanced Magitek as well. A blunder with Gate magic and Infinity's response to it made America's CIA aware of parachronics, and they've started spying on Homeline back (though Homeline is as of yet unaware of this); however, they are not necessarily hostile. More worrisome, the Condor Group Nazis in control of Argentina have made an alliance with Reich-5's Armanen.
Caliph is a mostly-peaceful world of Islamic Crystal Spires and Togas, but its technology is centuries ahead of anything that Infinity and Centrum have, and both sides want it. Of course, any intrusion risks telling Caliph about the existence of parachronics, and that centuries-advanced technology makes Caliph into quite possibly the most powerful single worldline in existence.
Most horrifying of all possibilities is Shikaku-mon. A dark Cyberpunk world, over half of which is ruled by a victorious Imperial Japan, it would be a likely threat to Homeline for that alone. What makes it worse, though, is that parachronic projectors work on that worldline, meaning that if they develop parachronic technology, they can reach Homeline. Also, the existence of Shikaku-mon is not a secret on Homeline, so it may be only a matter of time before the Masquerade breaks on this world as well. Sweet dreams!
Beware of Hitchhiking Ghosts: A double inversion. Troubled people might get picked up by a mysterious stranger who gives them life-affirming advice before dropping them off. Turns out he's some kind of supernatural being (not necessarily a ghost) who drives across worlds for this very purpose.
Broken Masquerade: The wheels are starting to come off in Quantum 3. Reich-5 is out of the bottle (though their parachronics are limited to a Government Conspiracy within the declining Reich) and has begun invading other worlds, while due to the emergency circumstances of first penetration, Merlin's U.S. government has discovered Infinity and is in the process of trying to infiltrate Homeline back.
Canon Welding: The 4e version of the setting has absorbed several campaigns from the 3e Time Travel book that were originally entirely separate settings.
In a more general sense, just about every other GURPS setting has a corresponding world mentioned in the book. Including the licenced ones (yes, Infinity agents can visit Discworld, Hyboria and Witch World). From a meta perspective, the Infinite Worlds setting basically exists to facilitate Massive Multiplayer Crossovers.
Crapsack World: Not as bad as most other cases and not immediately apparent, but the more you read, the more obvious it is that there are a LOT more crappy parallel worlds than reasonably nice ones (from the Lucifer parallels to the Reich parallels, with the disturbingly common Gotha parallels and many others in-between), and even the nice ones tend to have ugly caveats - Gernsback, for example, has world peace under the League of Nations (both The Great Depression and World War II were averted) and awesome Raygun GothicSchizo Tech thanks to Nikola Tesla's inventions, but due to not having had those experiences, racism, sexism and colonialism are still firmly entrenched at pre-1940's levels in most inhabitants' minds. It still is one of the most positive parallels overall.
Nergal, where the Assyrian Empire destroyed the Hebrews and Phoenicians, prevented the rise of Persia and set a very bloody standard for every empire to follow. Nearly every local culture practices human sacrifice, paper and alphabetic writing disappeared from history, and the world is slowly descending into an anomalous Ice Age that's hinted to be a result of dark magic. It's bad enough that Infinity has decided to throw The Masquerade to the wind; Nergal lacks the technology to replicate parachronics, and to quote the book, "frankly, the Patrol would like to cause as much cultural contamination here as possible."
Diminishing Villain Threat: In Alternate Earths, Reich-5's Imperial Japan, especially their Kempei Tai organization, were every bit as deadly an adversary as the Nazis. In Infinite Worlds, Japan is decaying into warlord states and the Kempei Tai have, within a single generation, gone from being the guys who gave the Gestapo nightmares to a bureaucratic tar pit.
The Reich itself plays with this; while the Nazi regime is no longer the unstoppable Orwellian terror that it used to be, a faction within the decaying Reich now has world-jumpers at their disposal.
Doing In the Wizard: Inverted, as there are several parallels where magic clearly exists, making it kinda hard to deny (though Centrum tries). There is even a loose group of powerful magic-users and monsters that also travel through realities, the Cabal (from Gurps Cabal, natch). However, the Merlin parallels play this straight by providing a scientific explanation for GURPS' default magic system, which is heavily tied in with parachronic travel itself.
Enemy Mine: one suggestion in the book is that Infinity and Centrum could ally against Reich-5. On the other hand, a vignette in the book suggests that Centrum might take advantage of "the Irrationalist sect from Bavaria" to draw off Infinity resources.
Equal-Opportunity Evil: Centrum is explicitly called out as finding the concept of discrimination based on race and gender very strange. They're a little more lenient on social divisions, but that's because they're meritocrats.
Evil Is Deathly Cold: Nergal's oncoming Ice Age appears to be a result of the gates of Hell opening. It's anyone's guess just how literally to take this.
Expendable Alternate Universe: Neither Infinity nor Centrum quite view people from other worldlines as real people. Among other things, it notes that some filmmakers have taken to filming real battles on other worldlines with thousands of people dying to use as footage in their historical epics.
For Science!: Apart from the Gernsback parallel which plays this straight (if mostly benevolently), there's an interesting variation with the Cabal.
From Bad to Worse: Many on Homeline in the know consider Reich-5's discovery of parallel worlds to be this; while their own version is, so far, much less performant than that of Homeline or Centrum, it could potentially end up forcing Homeline in a two-front conflict, as the latter's dimensional coordinates are between the other two.
Killed to Uphold the Masquerade: Stringently averted by the Time Patrol; Centrum has less scruples, but there are few situations where they'll bother (see just below).
Magitek: Exists on worlds where magic and technology have both been developed.
Merlin has the most advanced version yet seen, to the point where magic has been seamlessly integrated into the world's technological infrastructure.
Azoth-7 has a weird version, where alchemical developments have led to something that looks rather like Victorian science fiction, only a hundred years earlier and with a completely different form of technology.
Homeline's Paralabs have been trying to develop some, but it's mostly seen as a curiosity rather than a serious pursuit, as Infinity is mainly a technological organization. For example, they came up with a really weird conveyor that ran on Clap Your Hands If You Believe...and it and its inventor vanished in the first test.
The Masquerade: Infinity policy is to not reveal "the Secret" of parachronics to any outsiders, and to actively sabotage anyone else's attempts to do so; people who learn the Secret may be kidnapped and sent to Coventry. Centrum is less anal-retentive about it; they only care if the natives learn about them on planets advanced enough to develop parachronics. However, when that happens, they tend not to let technicalities (like the right to live) get in the way of business.
On Homeline, Infinity generally tries not to act like this...but there is no Alien Non-Interference Clause, and they're not against behind-the-scenes guidance of non-Homeline humans, or stealing technology and art from other parallels for Homeline's benefit. They also have to ride herd on sharp entrepreneurs who have their own ambitions:
Johnson Crosstime, Inc. ended up taking over Rome-2 because Infinity didn't read the fine print on his business license. By the time they realized what he was doing, the situation had become insane enough that they threw up their hands and ratified his buyout of the Roman Empire.
The Yugorovsky Group and The Mafiya send a lot of their members out to primitive worlds to "play czar," taking over nations to exploit them for their Homeline masters.
Infinity themselves have drawn up plans for the annihilation of the governments of Reich-5, in concert with Homeline's militaries. Operation Firefall was shelved because it was too risky and promised an appalling number of casualties even if Homeline won.
For some reason it seemed like a good idea to Infinity to put an alternate Cecil Rhodes in charge of mining operations on multiple Africas. There's no evidence he's become one of these yet, but some people in Infinity think it's just a matter of time.
N.G.O. Superpower: Infinity Inc. is probably the most powerful entity on Homeline economically and politically. Only a partial example, as Infinity is technically a U.N. member organization. (In practice, Infinity doesn't answer to anyone.)
Nice Job Breaking It, Hero: The snafu that resulted in the Reich-5 SS discovering parallel worlds and how to reach them was originally a Russian attempt to aid the partisans of the Reichsostland.
The "Gotha" parallels, and the plague named after them. The name came about because the first such world that Infinity discovered was just a particularly bleak Dark Ages parallel where Visigothic berserkers had destroyed every city in their path. It was only later that Infinity discovered that things were far darker than that.
A poetic-to-the-point-of-nonindicative name is the "Lucifer" series of worlds. No, these are not worlds defined by the presence of Big Red, they're worlds that have become or are becoming uninhabitable through a meteor strike.
Reed Richards Is Useless: Averted. Parachronics are making Homeline a better place in many ways...though Homeline's seedier elements are finding ways to use the infinite worlds to support their agendas as well.
Stupid Jetpack Hitler: Reich-5, a world where the Nazis have taken over and developed some superscience. They don't have world-hopping technology yet, but do have some innately gifted people who can do it.
Zombie Apocalypse: The Gotha parallels. Gotha zombies are carriers of a prion-based disease that mutated from the pneumonic plague, which retain some of their previous intelligence and memory, as well as just enough wherewithal to hold onto old cultural beliefs (usually with a strong slant towards fatalism and death worship). Thus why the zombies discovered on Gotha-1 were described as Visigothic berserkers; another Gotha parallel had zombie Thuggee in India.