In a small Midwestern town, Roger Phillips is found dead. When insurance investigator Sam Donovan arrives looking into the apparent suicide, all clues lead him to suspect murder. The man was shot, but there are no powder burns on the body nor was a pistol found. Unfortunately, no one wants to assist him with the case, including Sheriff Larry Best, despite a double indemnity clause. It turns out the dead man was universally hated. One of the prime suspects is Frank Baker, who eloped with Phillips' daughter the night of the death when Phillips objected to the marriage. During the investigation, Sam and local girl Anita Weatherby are strongly attracted to each other. This causes complications when Anita finds her father's hidden Luger pistol while searching for a place to store her diary away from her inquisitive younger sister. Meanwhile, at the annual town Christmas tree lighting, the residents are grieved to learn that beloved longtime physician Dr. Gerrow has died of a heart attack. Finally, Sam plants a false story in the newspaper, stating that he has sent for a chemist to find clues of the killer's identity from the floor of the house where Phillips died. The chemist is due very soon, so Sam waits for the murderer at the scene of the crime. Sheriff Best shows up first, but he knew the story was a fake. Next to arrive is Anita's father, Stu Weatherby, followed by Anita. Sam accuses Stu of the crime, but then realizes from the position of the murderer that he had to have been left-handed, which rules Stu out. Then Sam remembers, from his previous investigation, that Dr. Gerrow was left-handed. Stu confirms Sam's guess. Weatherby came upon Dr. Gerrow, just after the doctor snapped and killed the man responsible for so much misery to his loved townspeople. He talked Gerrow out of turning himself in and took the Luger away. Sam finally decides that the town will be better off if Gerrow's memory is protected, and agrees to report Phillips' death as suicide.
According to Madeleine Stowe, guest host of the May 21, 2016, Turner Classic Movies screening of Cover Up, the producer changed the time of the film, feeling the subject was unsuitable for a Christmas setting. Star Dennis O'Keefe, who also co-wrote the script, strongly resisted the change and won. This was the first film produced by O'Keefe's new production company, according to Stowe.
