Charles Oakley lives alone in a rooming house. One day, his landlady tells him that two men came looking for him; he sees the two men waiting on the street in front of his room and he decides to leave town.
Charlie Newton is a bored teenaged girl living in the idyllic town of Santa Rosa, California. She receives wonderful news: Her mother’s younger brother (her namesake), Charles Oakley, is arriving for a visit. Her uncle arrives and at first everyone is delighted with his visit, especially young Charlie. Uncle Charlie brings everyone presents. He gives his niece an emerald ring which has someone else’s initials engraved inside. Mr. Newton works at a bank and Uncle Charlie tells him he wants to open an account and deposit $40,000 ($0.6 million today) at his bank.
Two men appear at the Newton home posing as interviewers working on a national survey. Uncle Charlie is upset and berates his sister for opening up her home to strangers. One of the men takes a photo of Uncle Charlie, who demands the roll of film, because “no one takes my photograph.” The younger interviewer, Jack Graham, asks young Charlie out, and she guesses that he is really a detective. He explains that her uncle is one of two suspects who may be the “Merry Widow Murderer”. Charlie refuses to believe it at first, but then observes Uncle Charlie acting strangely, primarily with a news clipping from her father’s newspaper that describes a murder. The initials engraved inside the ring he gave her match those of one of the murdered women, and during a family dinner he reveals his hatred of rich widows.
One night, when Charlie’s father and his friend Herbie discuss how to commit the perfect murder, Uncle Charlie lets his guard down and describes elderly widows as “fat, wheezing animals”; he then says, “What happens to animals when they get too fat and too old?” Horrified, Charlie runs out. Uncle Charlie follows and takes her into a seedy bar. He admits he is one of the two suspects. He begs her for help; she reluctantly agrees not to say anything, as long as he leaves soon, to avoid a horrible confrontation that would destroy her mother, who idolizes her younger brother. Detective Saunders tells Charlie that the photo they took of Uncle Charlie was sent for identification by witnesses. News breaks that an alternative suspect was chased by police and killed by an airplane propeller; it is assumed that he was the murderer. Jack tells young Charlie that he loves her and would like to marry her, and leaves.
Uncle Charlie is delighted to be exonerated, but young Charlie knows all his secrets. Soon, she falls down dangerously steep stairs which she later notices were cut through. Uncle Charlie says he wants to settle down, and young Charlie says she will kill him if he stays. Later that night, she is prodded by Uncle Charlie to get the car from the garage. The engine was left running and the garage is full of exhaust fumes. She tries to turn the engine off but the key is not in the ignition and when she tries to leave she finds the garage door jammed and she is trapped in the garage. Mr. Newton’s friend Herbie happens to come by and hears Charlie banging on the garage door and gets her out in time.
Uncle Charlie announces he is leaving for San Francisco, along with a rich widow, Mrs. Potter. At the train station young Charlie boards the train with her younger sister Ann and their brother to see Uncle Charlie’s compartment. As the children disembark, Uncle Charlie restrains his niece Charlie on the train, hoping to kill her by shoving her out after it picks up speed. However, in the ensuing struggle, he falls in front of an oncoming train. At his funeral, Uncle Charlie is honored by the townspeople. Jack has returned, and Charlie confesses that she withheld crucial information. They resolve to keep Uncle Charlie’s crimes a secret.