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Parallel Worlds

Source Galaxy Exploration Manual pg. 143
Related Media: The Man in the High Castle (novel and TV series), Sliders (TV series), Star Trek’s Mirror Universe stories (TV series)
In a parallel worlds settings, heroes traverse parallel universes. They may be castaways, adrift through countless dimensions in search of their home, or perhaps they’re fleeing a cross-dimensional threat. The worlds they visit are usually different versions of one planet, such as their home world, but might instead be alternate versions of one or more Pact Worlds or some other interstellar civilization. For example, imagine a parallel universe in which the Swarm never invaded, so the war between the Veskarium and the Pact Worlds hasn’t ended. In another (or even the same) alternate world, the kasatha aboard the Idari might have followed through with their plan to settle Akiton and attempted to conquer the planet.
Regardless of what the setting is an alternate form of, the PCs are most often characters from different parallel worlds. In these cases, have your players create their own versions of their world as part of their backstory, take note of the different versions, and take the story back there, one at a time, to explore the setting your players have made, pursuing the story hooks therein.
One common version of the parallel worlds subgenre is a story with just two parallel worlds that the PCs consistently move between. All the primary attributes are exactly opposite in these two worlds. For example, one has high magic and is home to spellcasters, solarians, and vanguards, and the other has low magic and is populated by biohackers, mechanics, operatives, and soldiers. In one, the most common alignment is lawful evil, while in the other, it’s chaotic good.
How do the PCs travel from world to world, and how much control over their travel do they have? The easier it is for the PCs to travel from world to world, the more challenging the game becomes for the GM to run: if the PCs can hop worlds more than once per session, you’ll be improvising a lot of new worlds! Fortunately, the tables and other tools in this book will help you create worlds on the fly, and if you’re comfortable yielding some of the creative space to the players, they can help you create these parallel worlds as necessary. Another option is to create an interesting liminal space in which the PCs must spend a certain amount of time between visits to alternate worlds.
A parallel-worlds campaign is all about adjusting the attributes that describe a world, changing them from one game to the next. The PCs might travel from a world that is notable for high religion and accord, for example, to a parallel version that has low religion and accord. To the PCs, one of these worlds might be the “right” one, and changing the “wrong” one to be more like home is impossible. Instead, the PCs must survive long enough to escape or even do some good while they’re there. As the PCs travel from one world to the next, they might encounter distorted reflections of recurring NPCs whose alignments have changed to their opposite, so the PCs might help reflections of their enemies and battle reflections of their friends—or even themselves!
Parallel world stories can also come about from time-travel stories that lead to characters changing history; see Time Travel on page 145 for more on these types of campaigns.