Lots of people already got it right...major spoilers First off -- it truly is hard to review this movie without spoilers. So I say "screw it", it's spoiler time! Ok, first off, this is a great movie, but that being said, there are more plot holes than craters on the moon!
From the trailer, you already know that a family is being attacked by their "doppelgangers." And I must admit, the backstory is clever. The government created clones of people to see if those clones could be used to influence their counterparts. Ok, that in itself is idiotic, but that's not the best part...here come the spoilers --
The mother is the clone, but she chose to forget that -- she switched places with the "real" girl when she was about 10 years old. When the clone married her husband, her counterpart was "forced" to marry her husband's clone. Ok, that might be possible, but now for the parts that make NO sense. Both pairs had to children who were EXACTLY the same age and looked EXACTLY alike. That is, at best, 99.99% impossible...the only way that could happen is if when the couple on the surface had kids, the government IMMEDIATELY cloned them.
Ok, now that I've vented on the plotholes...this was a good movie, just don't think too much (like me).