It's present-day Earth. (The iPhone X has just been released, millenials love avocados, global politics are reliably unreliable, etc.) However, a race of 20cm-tall humanoids also exists on the planet. They're familiar with human culture, given that they've been living in our attics and using up all our free data, but humans didn't know that they existed.
These creatures have similar morals/drives/characters as humans, although they haven't had the same technology available to them, given that they've been living in hiding. They know how Facebook works, some of them are adept at coding, maybe a few of them post on StackExchange, but things have been either theoretical or pretty small-scale for them.
Question is, once we've all got over the shock of their existence, how will our societies coexist? Or, more fully, what could each society offer the other that makes coexistence worth their while?
(If it helps with visualisation, imagine a race something like the Borrowers, Wombles of Wimbledon, or house brownies...)
EDIT - MORE SPECIFIC
It's kind of irrelevant how humans and Minis come to coexist (I'm imagining Minis are discovered, and decide to live alongside humans peacefully rather than be exterminated by the more powerful Bigs), but I wonder what each society could offer the other to make their society feasible.
Perhaps the Minis would be really good at coding and needlework? Or their tiny size could make them really good neurosurgeons? And then humans could offer building skills, heavy lifting, mass-produced items, physical security..?
Basically, what else could they offer each other? I'd love some legitimate reasons for why their societies would actually want to live in harmony!
EXTRA INFO ABOUT THE MINIS
Physically, they're pretty much the same as humans (apart from size). They probably have better night vision, given that they're fairly reclusive - but you might also have some in abandoned warehouses/unpopulated places/etc. that have adapted to daylight more. They tend to recycle things as their communities are fairly small and fairly static, so they're quite ingenious and have good practical skills. They're also used to urban environments, and have access to technology in people's homes, so can be pretty adept with computers/phones/etc. They're fairly sociable, just a bit wary of the Bigs at first!