I picked this up due to all the great reviews.
The Earth has been taken over by nearly all-powerful aliens. After the takeover, the aliens capture a lot of children as a part of a boot camp of some sort. There, the kids are subjected to all manner of unpleasant training where the aliens treat the humans like garbage. Lots of children get killed as they are forced through adult-level boot camp.
No opportunity is spared at being unpleasant.
At no point did I understand WHY the aliens did this, aside from some odd prophecy. A single alien could kill the entire child army they are building. Yet they are spending extraordinary resources on all this training.
Jeff Hays is incredible, as usual.