The misadventures of Penrod Schofield, an eleven-year-old boy growing up in the pre-World War I Midwestern United States. His friends and his dog accompany him on his many jaunts, and they make names for themselves as "bad boys" who always have the most fun.

Focuses more on the relationship between the main character of the previous book, Penrod Schofield, and his best friend, Sam Williams, but they are not alone in their travels. They are accompanied by two other boys from across the alley, Herman and Verman.

Penrod establishes a detective agency and pretends to be detective George B. Jashber. In the process of solving mysteries, he finds himself getting into more trouble as a result.