This is the very first Judy Bolton mystery. We get introduced to the characters. Judy is 15. She's smart, but she's not as self-reliant as, say, Nancy Drew. She has an older brother, Horace, who she thinks is basically a coward. Mrs. Smeed is her grandmother. Both her parents are alive.
A dam is being finished near the town and she overhears an argument among two workers. Before long she's kidnapped, ending up in a room in a swampy area. She's released upon promising to her captors that she will not say a word of what she heard them talking about.
(This is a rough area of the book, I think, in that the two guys do kidnap her, but then they are not hardened crooks since they are willing to let her go on her own word that she won't talk about what she heard them say. Basically, this all relates to the dam being built in a shoddy manner.)
There's a scene where she's riding a horse that gets spooked by a train and she's thrown.
Judy continues to think of Horace as a coward while she finds out some more about the dam. There's a major storm coming and it's likely that the dam will break and destroy the nearby town. It turns Horace shows he's not as much of a coward as she thought.
Judy is, in some ways, a bit more believable of a person than, say, Nancy Drew. Judy doesn't always know just what to do. She's not as brave as Nancy and, at least in this book, she's not really as much of a detective. Yet she seems to react in a more realistic way a young girl like her would react if thrown into the situation she finds herself in.
It's an interesting first book, and the kind that leads people to want to read more of the series.