Maybe more like a 3 1/2. I enjoyed it. I thought it was funny, adventuresome, and just good fun. So for all those reviewers out there who poked holes in the story, thought it had today's world put in last century's setting, then yup, you're right. But it isn't meant to be super serious so just have fun with it.
Now, I won't say I wasn't surprised compared to what I thought the book would be about from the back cover. But once I let go of that, I relaxed and just chuckled along the way with it. To give a quick background- a contemporary doctorate student wants to learn the identity of the Pink Carnation, who is the "zorro" during the Napoleonic Wars (I couldn't think of any other comparison). So she finds a family who has a slew of documents that we read, along with her, about the Pink Carnation, the Purple Gentian and the Scarlet Pimpernel which is the bulk of the book. Yes, you're right, how she gets the entire dialogue between secret characters is irrelevant. It's just a story, people.
I enjoyed the frame of the story more than the story itself, but regardless, it was all fun. It has not changed my life, but it was fun to delve into.