It has dangerous moments but is told with a light hand, so that our sleuths never really seem like they could fail. To some this might be an annoyance, to me it is sometimes nice to feel you can totally depend on the author to protect their characters while writing an enjoyable story.
One of the things I really enjoy about Frankie and Bobby (our heroic duo) is how delightfully dense they sometimes prove to be. And why wouldn't they be? They stumbled upon a mystery and have no real idea of how to go about things other than what they've gleaned from detective stories. It'd be like you or I trying to go undercover and solve a crime.
Anyway, of course there's a couple who are friends but are destined to be more. There's a beautiful woman in need of help, and the plucky heroine has nothing but disdain for her. There's quite a few potential bad guys, and when the question is finally asked and answered there is a wonderful sense of adventure (I had an actual shiver when the question was asked, "Why didn't they ask Evans?", knowing that the answer was just a few sentences away).
In other words, this is another classically good Agatha Christie mystery (there's a sentence about how nobody looks at chauffeurs the way they look at regular people, so there's a bit of good old fashioned class based rudeness as well). It's lighthearted with some serious moments and instances of danger, and the solution is brilliants as always. I encourage you to read it. If not this one, one of the many other's she's left for us to enjoy.