Hearts in Darkness begins six months after Michael walked away from Nikki. Since then, he's been trying to regain control over his hunger. Seline has been helping him. She's been "a constant, if distant, companion. Badgering him endlessly, never letting his resolve slip, never letting him give up hope. She'd been his strength in the early days of darkness, when the demon was close to winning control, and he'd just wanted to give up the fight permanently." Nikki's powers have changed since Michael shared his life force with her. He hasn't told her that by doing so he made her his thrall. He has no desire to control her though, only to keep her safe.Nikki is working with Jake on their current mission, keeping an eye on Matthew Kincaid, a teenager who's mother is convinced he's going to run away with an internet girlfriend. She's also asked by a detective to help search for his niece. Her cases eventually lead her to Jackson Hole.Michael is dispatched by Seline to head to the same ski resort in Jackson Hole where people are going missing and "darkness is felt". She tells him "the darkness is linked to {his} past" and it's something he must deal with. He's resistant when he finds out that Nikki will also be there, working her own case, but finally gives in once she tells him that without his help Nikki will die.Together they head to the ski resort as husband and wife on paper while each deals with their feelings that have only grown stronger since being apart. Michael refuses all of Nikki's advances, telling her "My world is darkness, Nikki. Yours is light. We cannot be." No matter how strong his feelings are for her, he refuses to grow their relationship because of the fear that he will not be able to keep her safe. Nikki doesn't give up though, she keeps pushing at the walls he's built up to the point that Michael realizes he may never be able to walk away from her again.I enjoyed this story more than book 1, maybe because I'm now used to earlier writing style of Keri Arthur or maybe because of the relationship between Nikki and Michael. Either way, I continue to look forward to reading the rest of the series.