Psychiatrist Dr. Robert Ordway, known as the "crime doctor" because of his interest in criminology, leaves for a long-awaited vacation of hunting and fishing in the rural community of Brook Falls. Upon arriving there, Ordway learns from Jud Rookstool, the local storekeeper, about a mysterious malaise nicknamed the "summer complaint" that has proved fatal to several of the townsfolk. After Rookstool takes Ordway to the home of Doc Sam Millerson, where Ordway has rented a room for his stay, Millerson, an ignorant country doctor who practices folk remedies, expresses his hostility to modern medicine techniques. When Ward Beechy, the town barber, becomes the next victim of the "summer complaint," the police come to quarantine Brook Falls for typhoid fever, and Dr. Wickersham, the county health official, and Mort Crowell, the county prosecutor, convince Ordway to help them fight the epidemic. While studying some blood samples from the three most recent casualties, Ordway discovers that Ward's blood contains no trace of the typhoid bacterium, but does contain elements that suggest poisoning. When an analysis of some bitters consumed by Ward indicates the presence of poison, Crowell discovers that only two people had recently purchased poison from the local pharmacy: Millerson and Ezra Minnich, an illiterate exterminator. Ward had recently rejected Millerson's medical care in favor of the county health department, leading Wickersham to believe that may have provided Doc with a motive for murder. While Wickersham questions Millerson, Ordway interviews Ward's wife and learns that her husband was a womanizer who had affairs with both Rookstool's wife Eadie and Minnich's wife Ella. After an unseen assailant fires a rifle at Crowell, Minnich identifies Millerson as the shooter, but Ordway discounts his testimony because he knows that Minnich blames Millerson for his son Bije's retardation. The quarantine is ended just in time for Brook Falls's summer festival, and Ordway, hoping to expose Crowell's assailant, offers prizes for the shooting contest. After the contest, Millerson is handed a note by Myra Minnich, Minnich's young daughter, directing him to a meeting in the woods. Soon after he disappears into the trees, Millerson is shot in the back and killed. When Bije claims that he saw Rookstool discard the murder weapon, Sheriff Akers arrests Rookstool for murder. Believing Rookstool to be innocent, Ordway visits the Minnich house and learns from little Myra that she wrote the note for her illiterate father. When Ezra offers Ordway a glass of poisoned cider, Ordway accuses him of murdering Ward because of Ward's affair with Ella. After Ezra attacks Ordway, Myra runs crying to town to summon the sheriff. Accompanied by Crowell, the sheriff runs to the Minnich house, arriving just as Minnich is about to bash Ordway with a shovel. After the sheriff arrests Minnich, Ordway explains that Minnich killed Millerson because he was afraid that the doctor would remember his inquiry about how much poison it would take to kill a man, thus linking him to Ward's murder.