One night, a letter carrier appears in the bedroom of Mr. and Mrs. Blaha, who don't know what hit them. Mr. Blaha is told to acknowledge receipt. The contents of the letter tell him to go home and await further instructions. Suddenly, the doctor appears in Blaha's apartment on the third floor, claiming that he has also been called up for an "exercise". Blaha's confusion grows. He gradually learns that his apartment is to be "conquered" by the wine merchant Müller, who thinks Blaha wants to murd...