Paul Botts, the smartest kid in his group of friends, thinks that a local woman in his town might be a spy for an unknown foreign government. So he recruits his friends Quentin and the unnamed narrator, and the three of them decide what they're going to do about it.
Once thing to know is that this is not a book with lots of adventure, excitement, and danger. It's not like that at all. Instead, it's a pretty light-hearted take on what might happen if a group of friends decided to take on espionage themselves, albeit with the goal of bringing down a spy.
The first half of the book first deals with the problem of the alleged spy's annoying pet chicken, who is hated by her neighbor. If the spy's neighbor sues or pickets or does anything to draw attention to the spy's house, then the kids' own spying won't even work. So they take a train ride to New York City ("Is Paul going with you? It's okay then.") to try to talk to a lawyer about how to solve this issue. That goes about as well as you'd expect it to in real life, so the kids give up and decide to just go ahead and spy on their target anyway.
So the story moves on to basically the kids planning out their spying routine, as they decide on bringing snacks along and things like that, and then their actual spying. Various things go wrong, and the kids have to deal with them. Were they spotted? When their target gets out of her house, can they follow her and find out what she's up to? What about when that annoying chicken starts pecking at the bush they hide behind?
Well, in an ending that fits naturally with the story's mood and theme, but which made me feel cheated when I originally first read it, the woman turns out not to be a spy after all. Disappointing originally, but it does cement the book's theme. See, this is not some exciting story about kid espionage with danger and threats around every corner, but rather, a more grounded, down-to-earth story about what might happen if three average kids decided to engage in some espionage of their own. As a more casual take on a theme I'd always liked, it's pretty neat.