This was okay. The mystery was solid, but everything around it felt a little haphazard. Lady Rose is an outspoken, intelligent young woman who is ahead of her time (which is the Edwardian era). Captain Harry Cathcart is a young man invalided out of the army and at loose ends. When Rose's father is concerned about her suitor, he hires Harry to investigate. Harry unearths a scandal, which, although completely the fault of the cad courting Rose, makes Rose unmarriageable. Rose resents Harry; Harry thinks Rose is beautiful, but unwomanly.
And then we get another fifty pages of meandering where it's established that Harry is now a private investigator, and Rose is still unmarriageable. The actual plot picks up when Rose is invited to a house party and one of the other guests dies from arsenic poisoning. Rose's host the Marquess would like this to be an accidental death, but Rose thinks otherwise. The Marquess calls in Harry, this time to cover it all up, but he's too honest and insists on investigating, with Rose's help, which he doesn't want.
Chesney knows how to tell a mystery, so it's not unreadable. It's just really hard to warm to Rose. Normally, when you get a character who's told she's unwomanly and needs to learn to flirt and so forth, you know that's code for someone who refuses to be defined by society's unfair rules. But Rose really does come off as unsympathetic, hard, abrupt, and basically unlikeable. Yes, telling her she needs to learn to flirt is probably wrong, but she does need to learn to be more pleasant. I have a hard time believing Harry is attracted to her, nor her to him. Harry, for his part, has some hints of an interesting character, but rarely rises above the typical.
There's also a too-heavy dependence on the disparity between Edwardian manners and actual Edwardian behavior, but this inadvertently promotes a commentary on women's roles, so I was willing to overlook this. I was very fond of Harry's manservant and Rose's maid (a former actress with hidden depth) and would like to see more of them. But with Rose as unsympathetic and Harry as underdeveloped as he is, I'm not sure I'll continue with this series.