A distracted professor, flea animal expert Loris, gets his overcoat interchanged by Lieutenant Juncker during a restaurant visit. Junker discovers the mistake during a night grog with two joke-loving friends. He finds the professor's notes for the next day in the professors coat and bets with his cronies that he should be able to perform the professor's daily program without being revealed. Juncker visits a hairdresser and gets himself made up as Professor Loris. Then he goes to a gymnastics institute to fulfill the professor's first program point, morning gymnastics. This causes a great tumult because Juncker very is much fitter than the professor. Then the disguised lieutenant goes to the university and manages to give lectures to the students about the importance of the flea in the world war. Here, however, the right professor appears and Lieutenant Juncker has to slip away. The day continues according to the professor's notes, and constantly the lieutenant is followed by Loris, but at no time he is revealed. However, the real professor's niece, the beautiful Ina, becomes suspicious when he brings her flowers. Finally, the lieutenant has only one program item left to win the bet, a visit to the professor's aunt. He knows the address but must find himself to be driven out from several apartments before he finds the real aunt. Here, all the people of the piece converge and the joke is revealed to everyone's humorous satisfaction. The lieutenant and Ina may become a couple.