Nobody he knows ever said Stanley Hastings was the brightest PI on the street. In fact, they usually call him "moron." Even he acknowledges his failures at acting and other endeavors, and his main source of income as an "investigator" for a negligence attorney requires little intelligence. In fact he is the only one filling the position who doesn't leave after a very short time.
So it is no surprise (in this or any other novel in the series) that Stanley is taken in and bumbles along some convoluted path until whatever trouble he finds himself in is resolved. In "Caper," he is retained to find out why the client's daughter is skipping school. Well, of course, everything is not as it seems, and at one point a murder complicates Stanley's path to solving the "case." He even becomes a suspect.
Strangely enough, as one reads, Stanley emerges less as a fool and, perhaps, more as an idiot savant. There are major portions of the novel that are very funny. And more important, in today's often dreary world, it is fast reading and an interesting tale, and is recommended.