The movie opens on a man keeping a seven year old girl in a house. He insists they stay inside and never look out, and that she practices memorizing the names and behaviours of an identity not her own. She's rebellious and wants ice cream very badly - and there's always an ice cream truck just outside the house. Inevitably, she sneaks outside the house - and things get even weirder.
If I tell you much more than that, I get into the realm of spoilers. I usually take hints about spoilers from the trailers, because if you want to watch the movie you've probably seen the trailer and know what's said or implied there. Which means it's relatively safe to say that this is a near future science fiction superhero movie, although "superhero" implies they're good guys when all I can say is that the main characters are super-powered. And hunted, as "freaks."
I actually watched the trailer after the movie - and was interested to see that another reviewer had made the connection I immediately made, which was to "10 Cloverfield Lane" - they called it a "mystery box" movie, which seems like a good name. In both cases, you're in claustrophobic circumstances with possibly deranged people and you have no idea what's going on until, over the course of the movie, the truth of the world is revealed to you.
The movie is creepy, disturbing, and quite good.