So, Mark Braglewicz, private inquiry agent, has identified Pen and Greta Starling, of the Flying Starlings trapeze act, as the legitimate children of a nobleman who hid his first marriage and committed bigamy. That makes Pen the heir to his title and estate. But Pen has never felt like he fits in a box marked "man" or "woman" - or at least, not like he can stay in one of those boxes permanently - and can't see a life for himself outside of the music hall, where he is free to present himself as masculinely or femininely as he wishes. (Note: Pen's pronouns in the book are in fact he/his/him, so I'll use that rather than defaulting to a singular "they" that Pen never actually chooses.) But there's a killer on the loose, and if the identity of the lost heir remains secret, his life might not be the only one in danger.
This is an excellent conclusion to the series. Earlier books in the trilogy were a bit slower paced while the tension built. I enjoyed them, because I enjoyed getting to know the characters, but by the time we get to An Unseen Attraction, the pieces are all in place. We still spend some time getting to know the characters, but the intrigue and the danger from the ongoing plot is looming in the background from the beginning and present all the way through. Then there's the romance. Mark says at one point that he's trying not to make assumptions, and it's true. This is the antithesis of so many romance cliches, from the conflict caused by one or both members of a couple refusing to talk directly to the other about their problems or fears to the magical ability of a new lover to know exactly how to please the other in bed. Their communication is beautiful and is the embodiment of sexy enthusiastic consent.