This is the final book in the Circle Opens quartet. It follows Tris and her teacher Niko on their adventures out in the world. Like the other three books in the series Tris discovers and untrained ambient mage and has to take him under her wing. Unlike the other books, he's had a little magical training (meditation mostly) and he shares an aspect of her powers - lightening. Over the course of the book he and Tris help police to hunt down a serial killer in the city.
I was glad to see a return to the student being integral to the mystery in the book. That was a problem with book three - the students felt like they weren't even part of the story. I enjoyed watching Tris try to teach a student with a temper and willfulness to match her. The mystery was a bit lacking, as there was no way to guess who the killer was before the reveal. But as this isn't a true mystery novel, I'm willing to let that slide.
I enjoyed this book while reading it and felt that it wrapped up the quartet very nicely.
With the completion of this book The Circle Opens series is done. It's a well written and exciting story. Tris is a delightful character, even more so as this fourteen-year-old finds herself teaching an adult how to control his magic. Of course, it's hard for Keth to accept that a child is teaching him about magic but she has the qualifications and skills to convince him. He needs to control his magic and together they need to catch a killer who is preying on beautiful female entertainers.