After bringing the corruption of her former boss at the San Francisco office of The Protectorate to light, and getting him fired, Alma Bellrose is dismayed to learn that a famed demon hunter will be replacing him. She knows that they keep tabs on her and the changeling she once saved from execution, Sebastien, so to distract The Protectorate she agrees to go undercover at a week-long party hosted by well-known witches. Right off Alma knows something is wrong when she doesn't see any fae on the property, a sure sign that a "demon", or more correctly, a possessing spirit is present. But seriously, all of these people are behaving suspiciously, except the actual "demon," and it's not long until the bodies start to pile up, starting with the hostess apparently committing suicide.
When she refused to kill the changeling Sebastien without proof of wrongdoing, Alma knew that The Protectorate's stance on possessing spirits, kill them all, was not aligned with her own. It was wrong to switch the souls of two babies, but the crime was committed by Sebastien's mother, not the changeling himself. Now she meets another example that is closer to the classic profile of the possessing spirit as a "demon" in that he has amassed a great deal of money. The other classic sign is someone who builds up power. But aside from being a demon in the board room, Phillip is a loving husband, a philanthropist, and he even keeps his business dealings strictly legal. Alma decides Phillip didn't use evil powers to have their hostess drown herself but someone else disagreed. He's staked with silver and burned, the traditional way to kill a demon, and Alma isn't sure if they are now dealing with one or two murderers.
Not only does this story dig into the different kinds of possessing spirits it also exposes Alma's most closely guarded secret to Damien, that she can see the fae without any magical aids. She's never heard of anyone with that ability and doesn't know what The Protectorate would do with that knowledge but she doubts it would be good. Unfortunately, someone else figures it out and because they actually have heard of that ability and know what it means they are determined to kill Alma too.
This is probably the most important book in the series. While Alma is slowly getting roped into doing favors for The Protectorate she's also learning more about the various kinds of possessing spirits, which is an important aspect of the world building. She discovers that she's not alone in her ability to see the fae and that it could be deadly if the wrong people find out about it. Alma also learns about a kind of magic that can control a person's actions, which is definitely evil when it forces someone to commit suicide, and yet another thing she needs to hide from The Protectorate. Can she trust the her ex-partner Damien?