Nick Charles, the famous New York detective, takes his wife Nora and their dog Asta to his sleepy hometown of Sycamore Springs to visit his parents and celebrate his birthday. Soon after they arrive, Nora discovers that Nick's father, Dr. Bertram Charles, is disappointed in his son for not having become a doctor as he did. Determined to show Bertram that Nick is an accomplished sleuth, Nora encourages Nick to find and solve a crime in the small town. When a rumor begins to spread through town that Nick is visiting Sycamore Springs to investigate a case, some of the residents, especially Edgar Draque, who is secretly involved in an espionage ring, become alarmed. Draque tells his wife Helena that they must leave town immediately after they get their hands on a particular painting by local artist Peter Berton. Meanwhile, Nora, who is looking for a gift for Nick's birthday, finds the Berton painting in Willie Crump's art store and buys it for Nick. Later that night, Berton arrives at the Charles house and is about to reveal some important information when he is struck by a bullet and killed. While Dr. Bruce Clayworth, the town coroner, performs an autopsy on Berton, Nick begins an investigation into the murder by going to Berton's residence at Tom's Auto Court. There he learns that Berton had a fight with someone earlier in the week, and that the fight may have been over Laura Ronson, the daughter of banking tycoon Sam Ronson and the girl friend of Tom Clayworth. Inside Berton's room, Nick finds a rare Cuban cigar wrapper and continues searching for clues until Crazy Mary, a local character, enters the room and knocks him unconscious. Nora later shows Nick the painting she bought, but when he tells her that the windmill in the painting brings back bad memories for him, she donates it to the upcoming charity bazaar. Nick continues his investigation with a visit to the Ronsons. There he notices Cuban cigars wrapped in the same wrapper he found in Berton's room. When Laura tells Nick that she knows nothing about Berton's fight, Nick reminds her that her father is known to have objected to her friendship with Berton. R. T. Tatum, one of Ronson's employees, later warns Nick to stop meddling in Ronson's business and threatens to spoil his father's plans to open a hospital if the investigation continues. Nick, however, ignores Tatum's threats and later learns that Mary is Berton's mother. He also learns that Mary gave her son up for adoption when he was a child and that Berton never knew she was his mother. Meanwhile, Draque traces the Berton painting to Nora and offers her $500 for it. Nora, realizing that the painting must be of some worth, quickly loses Draque and takes Nick with her to the bazaar to look for the painting. At the bazaar, Nick and Nora discover that the much sought-after painting was sold to Helena and that Ronson is among the many who have been searching for it. When Nick enters Helena's hotel room, he discovers that she has been knocked unconscious and that the painting is missing. The trail of evidence leads Nick and Nora to Mary's. There, they discover Mary has been murdered, but before they leave, Asta uncovers the hidden painting. Believing he has the evidence he needs to solve the case, Nick calls the police and assembles all the possible suspects at his parents' house. Nick explains to the group that a special instrument has revealed that several of Berton's paintings concealed top secret plans for an airplane propeller manufactured by a company owned by Ronson. After exposing Draque's involvement in a scheme to sell the valuable plans to foreign interests, Nick tricks Dr. Clayworth into revealing himself as the killer. Clayworth desperately grabs the rifle used to kill Berton and points it at Nick, but because Nick has anticipated the move and removed the firing pin, the gun does not fire. The police arrest Clayworth and, to everyone's amazement, the crime is solved.