This hypothesis is popular among conspiracy theorists, and among the general public. It’s the theory that aliens have already come into contact with humanity, but we don’t know about it yet. Maybe no-one on earth knows about the extra-terrestrial presence, or maybe only a select few do: world leaders, scientists, et cetera. Whatever the case, aliens are already here, already on Earth, and already watching our civilizations grow and develop.
The potential premises that arise from this hypothesis should be obvious. Stories where aliens are living among mankind could focus on the aliens and how they integrate into society, if they’re even able to, how they hide among humans, what their purpose is, et cetera. It could focus on a human, or a group of humans, learning the truth about aliens living on earth (for good or for bad), or it could focus on aliens and humans working together while they prepare to reveal the truth to the world. The choice is yours.
Citations:
Speigel, Lee. “Survey: More People Believe In Space Aliens Than In God.” HuffPost, 50:36 400AD, https://www.huffpost.com/entry/alien-believers-outnumber-god_n_1968259.
Fermi's Paradoxical Prompts 