I know it's not fair to compare this book to Zero at the Bone, but, I can't help it. It's sort of a similar storyline. Where I think Zero was a bit more ambitious, in the end I thought it bit off more than it could chew. Shades of Gray takes a smaller bite and is a bit more graceful about it as a result.The book opens with Danny Butler sweating, and bleeding, it out while waiting alone in a police interrogation room. A cop pulled him over for running a red light and bags him on a weapons charge, dragging him through a broken window in the process, leaving him bloodied and looking at a long stint in federal prison. He's a career criminal. In the fourteen years since he joined Roberto Hinestroza's international drug smuggling enterprise at 18, he's risen up the ranks and is currently a mid-level drug runner. He's no stranger to the federal prison system, and he's not at all eager to head back, but he's even less eager to roll over on Mr. Hinestroza and take a deal with the FBI. Not only is he afraid of getting killed, he has a sense of loyalty towards the drug lord that he's not ready to abandon.FBI Special Agent Miller Sutton wants Danny to take this deal, and he's not above playing dirty to get what he wants. He's been on Hinestroza's case for three years, following Danny for over half a year, so he pushes and pulls to get what he wants, implying that he'd let Hinestroza think Danny's working with the Feds even if Danny turned him down. He knows he's being a dick, that he's placing this guy in mortal danger, but that's just what he does. He's with the Good Guys. Sad for Miller that he's not sure he believes his own propaganda anymore. He wins a deal from Danny, so why doesn't he feel like a winner?Though the plot is centered by Danny informing on his boss, outrunning a hitman and the like, it's not really a romantic suspense. It's more of a character study than anything else. As Miller babysits Danny in a secure apartment, they talk and form a friendship. Approval-starved Danny is attracted to the good in Special Agent Miller Sutton. Closeted Miller, stressed out by living a carefully guarded lie, is mesmerized by the impulsive, comfortably gay Danny. They're clearly attracted to each other early on, but just how would a FBI agent and a drug runner hook up?I found the romance gritty, but entirely realistic. Not in the way that this is how any of my friends have met their better half, but that if an FBI agent were to hook up with an ex-felon informant, this is a completely plausible how. I mean, what they're doing breaks all their rules. So why? Why these two? Why now?McKinley's superior characterization answered those questions for me. Through dialog, POV prose and bits of deep POV flashback, we get the full picture of both lead characters and what made them who they are today. Both have a character arc that spans the length of the book. Both are sympathetic, but with flaws and bad decisions aplenty. At the beginning, it seems a bit like Miller is the good guy and Danny the bad guy. But the more we find out about the both of them, the more decisions they make, the more those lines blur.It's hard to offer examples of the characters without giving away crucial bits of plot. Both plot and character are closely intertwined and paced perfectly. Suffice to say, I could see how Miller could feel so overwhelmed by his life perched upon one lie after another that Danny was the straw to break the camel's back and make him throw everything away and start over. I could also see how maybe Miller needed to do it this way, so that he'd have no way to go back to how he'd been living before even if he wanted to. Those actions and decisions, I thought, turned him into a man only someone like Danny, who is well acquainted with how easily people can screw their lives up, could understand. By the end, Danny has begun to acknowledge the good inside him and live with it, while Miller is just getting used to living with his darkness.I'm doing a bad job of reviewing this. I can never decently review a five star book. They've got a magic quality to their storytelling that I can never quite adequately describe. But, to sum it up, this was a great character-driven romantic suspense in which the characters personalities, gender and sexuality all played a major role in the plot. Both men acted like men; neither could have been replaced by a heroine and kept this plot. It was a story that could only be told within a m/m framework. It's a rare book that makes me want to re-read it so soon after finishing it.