Gunnar Bergman, a detective sergeant in a small Swedish town, is called in to investigate the death of a resident of a shady boarding house. He makes the mistake of telling his father, Carl, all the details of the case, including the fact that the victim’s room was locked from the inside. Carl, a locked room enthusiast, is a member of a small club which meets regularly to discuss locked room classics. He and his fellow members gleefully seize on the case and apply chains of Ellery Queen style logic to devise a solution which Gunnar finds so hilarious he reads out extracts to his wife in bed. Meanwhile, Gunnar himself interprets the same set of clues to arrive at a totally different solution, through shoddy police work which follows the path of least resistance. It is left to a cheese-loving local doctor to interpret the clues in yet another way to solve the case and confront the murderer.