This is the first book in the Buan series, and as such, it is largely setting up the story. In a world of magic and demons, the lives of four siblings change forever as each of them gain magical powers and are sent on a mission to save the world. (That is basically the entire plot so far).
Not much actually happens in The Perfect Mortals. We meet the main characters and see them gain their powers and learn how to use them. It all kicks off in the last chapter when they fight a bunch of monsters and start to really understand the peril that lies before them.
There isn’t a huge amount more to say. Bridger’s writing is good and the plot has the potential to get much better as the books continue. This could turn out to be an epic fantasy series (although, from the pace of the first book, I doubt it will be anywhere near as good as the likes of Eragon or His Dark Materials). I have high hopes for book #2.