A tabloid newspaper reporter overhears a conversation about murder between the owner of a shooting range and his niece Slávka, an avid reader of detective stories. The evidence in the case was underwear with monogram I.K. Professor Sedláček returns from Paris. His wife welcomes him fondly but when she discovers his travelling case is full of ladies' underwear, the idyll is over. But she does not want to get divorced because she loves her husband still. She just wants to punish him adequately. On the advice of a lawyer she decides to arouse his jealousy by flirting with other men.