From what I can gather from the trailer, it's set in the near future after we've overbreeded ourselves on Earth, and so we create a colony ship to send out to find "a new home, a new beginning for Mankind" so that we can destroy that one too.
From the Gamespot article:
Cast upon some unknown world, you will encounter alien life, push the limits of humanity, and inevitably come to blows with your fellow man over whose vision of the future is "correct."
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Before you land on a new planet and start shaping humanity’s destiny, you must first equip your expedition. Referred to as "the seeding," this is an expanded version of your civilization selection in previous games. In Beyond Earth, not only do you choose which faction (read: civilization) to play, but also which spacecraft to take, what cargo to carry, who to bring, and the type of planet you want to inhabit. Each selection you make will greatly impact the start of your game: you could carry high-quality cargo that grants additional funds up-front, for instance, or hard-working, production-oriented colonists who build structures faster. The Civilization series has always been about defining a strategy early on, and then adapting that strategy to accommodate whatever curveballs the game throws your way. These choices should give you greater control over how you define your opening gambit.
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When other players start showing up, it will most likely result in open warfare. It was at this point in the interview that I asked whether Beyond Earth would bring back unit stacking, or continue using the one-unit-per-hex rule introduced in Civilization V. According to lead designer David McDonough, Beyond Earth will not have military unit stacking. "I am a big fan of the way warfare was designed in Civilization V," McDonough added. "I thought it was very elegant and would make a good fit for this game as well considering we’ve added these alternate game layers--such as the orbital layer--which sort of let you break that rule by launching units into space and having them effect units on the ground without being stacked on top of them."