Although there is a minor sub-mystery -- the case Charlie passes off to a co-worker before heading off to Elysia -- the main mystery thrust of the story is saving Hank from being punished by the Circe.
Like most series that continue to follow the same characters, books like this -- where one of the main characters is so intimately involved in the mystery -- are my favorite. All the characters and all of the events are part of the core story. There is no "filler", no excess.
The story starts on a sour note. The ambassadors to Elysia have been told that Hank was killed immediately upon arriving back in Fiallan. Since Charlie already has the time off and permission to enter Elysia, Emma and Rex convince her to go anyway. And, of all people, Sandra the oracle is waiting to accompany her.
With Sandra along, Charlie plays security guard and companion, while Sandra negotiates an audience with the Circe directly. An invitation is so much easier than sneaking around the city hoping that the mark Hank gave her will warm.
This is possibly spoilers, so if you're sensitive, avert your eyes.
I was happy that Charlie found Hank before the end of the book. I was worried about either another cliff hanger or a rushed ending. But she found him early enough in the book for us to see some of the consequences of his imprisonment. Not to mention a goddess blessed oasis for the both of them to recover.
We also learn a lot more about the shared history of Elysia, Earth, and Charbydon. We learn of the First ones and their Disciples, who were seeded onto the three worlds, and the indigenous people who were in their "cave man" phase when the Creator seeded the worlds. The sirens, fae, and nymphs of Elysia, as well as the jinn and ghouls of Charbydon were all born of "the primal deities and energies that emerged in those worlds' infancy".
All and all, an excellent example of urban fantasy. Unique and fascinating world building. A seemingly impossible quest. Self-sacrifice on a number of fronts. A budding friendship (with Sandra). A partnership that is on the verge of something more. A satisfying ending that leaves possibilities open for the future. A tease of romance. (And, yes, I wanted more.) All signs that romance won't end the firm foundation of friendship that came first.
A delightful morsel.
Oh, and a technical point I wanted to make. The book is written in first person from Charlie's point of view, with some brief passages from Hank's point of view. I really appreciated the italics used to identify Hank's point of view. I frequently get lost between narrators when the POV switches too often, but this was smooth as silk.
I'm back to "no complaints except the next book isn't available immediately", and a tad bit of "wish there was more on the sexual front". But, hey, any more and the book wouldn't be as tightly written.
Kudos to the author. My favorite in the series, thus far. One of my favorite urban fantasy series, at the moment.