Nick and Nora visit Nick's parents in Nick's hometown, Sycamore Springs, in New England. The residents are convinced that Nick is in town on an investigation, despite Nick's repeated denials. However, when aircraft factory employee Peter Berton seeks out Nick and is shot dead before he can reveal anything, Nick is on the case. An old childhood friend, Dr. Bruce Clayworth, performs the autopsy and extracts a pistol bullet. Then, when Nick searches Berton's room for clues, he is knocked unconscious by Crazy Mary, a local eccentric. Nora's innocent purchase of a painting for Nick's birthday present turns out to be the key to the mystery. When she shows it to her husband, it brings back unpleasant memories for him, so she donates it to a charity bazaar. When Edgar Draque offers Nora a large sum for the painting, Nick wonders why it is so valuable. Nick learns that Draque's wife Helena bought the artwork, but she is knocked out and the painting disappears. Nick discovers that Crazy Mary is Berton's mother and goes to see her, only to come across her lifeless body. Nick and Nora's dog Asta finds the painting in her shack. Nick puts the pieces together and has the police bring all the suspects to his father's house. Using Dr. Charles's fluoroscope, Nick shows that there is a blueprint hidden underneath the paint. Several people identify it as part of the specifications for a new aircraft propeller worth a great deal to a "foreign power". Berton had copied the blueprints and concealed the copies under five paintings. He had a change of heart and was going to confess all to Nick, but was killed by the spies he was dealing with. Nick has a souvenir World War II Japanese Nambusniper rifle belonging to Dr. Clayworth's brother brought in, and claims it was the murder weapon. Then, after proving that the Draques are members of the spy ring, Nick reveals the identity of its leader: Dr. Bruce Clayworth. Clayworth's first slip was the bullet he showed Nick. Nick knew a handgun bullet would not have the power to penetrate as far into Berton's body as the real one went. Clayworth grabs the rifle. He confesses to the murder, and also to a deep hatred for Nick for always being better than him in their youth. He tries to shoot his nemesis, only to find that Nick had taken the precaution of removing the firing pin. Nick's father is very impressed.
