Professor Albert A. Topaze, an honest, naive chemist and schoolteacher at the Stegg Academy in Paris, loses his job when he refuses to alter the grades of Charlemagne de La Tour-La Tour, a bratty baron's son. On the same day, Friday the 13th, Topaze calls on Coco, the baron's mistress, who is shopping for a tutor for her sister's child and has gotten Topaze's name from La Tour. Upon meeting and listening to the sincere remarks of Topaze, the baron, who is the head of the La Tour Chemical Works, decides to employ him as a scientific dupe for his phony curative water. Unaware that the water, "Sparkling Topaze," which is being sold all over Paris, does not contain the medicinal formula he invented for it, Topaze is shocked when a blackmailing colleague, Dr. Bomb, accuses him of being a thief and a fraud. After confirming that "Sparkling Topaze" is in fact phony, a dazed Topaze returns to Coco's apartment ready to be arrested. Instead, however, he is awarded the Academic Palms prize by a corrupt government committee. His naivete thoroughly destroyed, Topaze decides to fight back by becoming more corrupt than his mentors. With Bomb as his assistant, he opens his own office, where he makes dignitaries wait to see him and then blackmails the baron into a partnership in his company. Because of his newfound prestige, Topaze, who has also garnered the romantic attention of Coco, is asked to speak at the Stegg Academy graduation. Instead of giving Charlemagne his ill-gotten class prize, Topaze, suddenly re-inspired by his own philosophical placards, makes a speech about life's pitfalls and awards the prize to the entire class, admonishing them to remember the importance of ethics and honesty.