I'm not a huge sci-fi fan, but I quite enjoyed this one. There weren't too many details on the alien/vampire other than that they took over and were prowling for the humans. With the food supply dwindling, they took to breeding them for food. Again, there aren't too many details on this, just that it is going on.
Shylar is an alien/vamp who has escaped and is essentially looking to defect. His plan is to save Angus and help defeat the aliens/vamps. Shylar and Angus fall for one another, but in the middle of that is a vengeance seeking bad guy and the humans inability to accept Shylar isn't one of the bad guys. There are more details here, but any more would give too much away.
There was a really nice mix of just what I needed to know to keep it interesting, but not too much to make me lose interest. Overall, this was a really enjoyable story.