This game is freeform, and draws its inspiration from the movie They Live (although the movie is in no way canon to this story, and there will be some differences). While by strict definition this is a sci-fi story, the space opera aspects of it will actually be turned down rather low. It will, at its heart, be speculative fiction, set in the modern era, drawing from the history of the real Earth where technologically advanced civilizations colonized primitive societies...and exploited them.
What if the same happened to Earth? Only instead of landing with ray-guns blazing and starships hovering over the great cities of Earth, the Colonizers--Them--subtly subvert the existing authorities of the world, bending the great human powers to Their secret will, gradually reshaping human civilization to serve Their interests? 99.9% of the human race don't even know anything is going on. Those who do are dismissed as kooks, as conspiracy theorists, ridiculed, scorned by the media.
Or, if they actually start to draw a following, they just disappear.
Your characters are only unique in that they, for whatever reason, have learned the truth--or at least a part of it. What they do with that knowledge is up to them. But the time has come to choose sides. For soon They will advance Their plans for humanity, and those plans are very ungood.
One hint: They are remarkable like us, humanoid, mortal, with the same lusts, the same goals, the same sins. The only real difference between Us and Them is that They, unfortunately, have seniority...and most of the planet is asleep.