1940, World War II. In the southern English town of Hastings, as the war intensified, antipathy toward refugees and exiles from enemy Germany grew stronger. Then the German wife of the local magistrate of public safety dies a tragic death. Superintendent Foyle, accompanied by his personal driver Sam, begins an investigation.

As the threat of German invasion grows, Foyle makes contact with Nazi supporters in England. One day, during an anti-Semitic meeting at a hotel, the hotel manager is murdered. Foyle detains everyone present at the scene and begins an investigation, which uncovers a major conspiracy.

Foyle begins investigating the case of a young man who was arrested and tortured to death in a detention center after he petitioned the court to deny him military service. At the same time, a threatening letter is thrown into the home of the judge who rejected his application. Foyle investigates the judge's surroundings and arrives at a truth.

[Season Finale] A man's body is found in the burnt ruins of an air raid. Foyle begins an investigation based on a pendant the victim was carrying and information from witnesses. He also uncovers a plan to steal artwork to be transferred from a museum. Meanwhile, his son Andrew is assigned to a department in the Air Force that handles the latest radar technology.

In September 1940, amid frequent German air raids, Sam's boarding house is bombed, and in the chaos, the landlord's valuables are looted. At the same time, a guest from the U.S. is visiting Foyle's acquaintance's house. He is a key figure in the enactment of the Arms Lending Act, under which Britain will receive aid from the United States.

A man Foyle had once arrested overturns a truck with a large amount of oil in it and dies. At the same time, a large amount of oil was missing from a storage facility. As the investigation continues, a woman who worked there is found dead in her home. The woman was dating an ace Air Force pilot who was a close friend of Foyle's son, Andrew.