At a time when German nationals are being interned in British camps, the beautiful Austrian wife of a local magistrate is resented by locals. When she is found murdered, a swastika painted on a nearby tree, Foyle realizes the seemingly peaceful Sussex town harbors any number of suspects.

The arrest of a young girl on a sabotage charge brings Foyle into contact with a group of English Nazi sympathizers. Shots are fired at the group's charismatic leader, but it is the hotel owner who dies. Foyle investigates the murder and uncovers a plot to smuggle a vitally important letter to the enemy.

A conscientious objector dies in police custody, setting in motion a series of suspicious deaths and a complicated puzzle for Foyle. When Foyle has pieced together the truth, the entry of Italy into the war engulfs a close friend in tragedy.

In the course of investigating a murder, Foyle uncovers a plot to steal art treasures being moved from the local museum to Wales for safekeeping. Meanwhile, Foyle's son flies his first mission as a Royal Air Force pilot and is accused of passing top secret information to the enemy.