Lawyer Francis Pettigrew takes a job at the Pin-Making Office. To keep from withering away from routine, the staff, in their free minutes, amuse themselves by writing a scenario for the perfect crime.
But reality soon outpaces their fantasies: a spy is discovered in the department, and shortly thereafter a murder occurs that, at first glance, has neither a reason nor any benefit.
The police find themselves at a dead end—and Pettigrew, relying on the “theories” of amateur detective lovers and on his own powers of observation, decides to solve the case himself.