Mr. Topaze (Peter Sellers) is an unassuming school teacher in an unassuming small French town, who is honest to a fault. He is fired when he refuses to give a passing grade to a bad student, the grandson of a wealthy baroness. Castel Benac (Herbert Lom), a government official who runs a crooked financial business on the side, is persuaded by his mistress, Suzy (Nadia Gray), a musical comedy actress, to hire Mr. Topaze as the front man for his business. Gradually, Topaze becomes a rapacious financier who sacrifices his honesty for success and, in a final stroke of business bravado, fires Benac and acquires Suzy in the deal. An old friend and colleague, Tamise (Michael Gough) questions him and tells Topaze that what he now says and practices indicates there are no more honest men.

El señor Topaze es un profesor que pierde su trabajo porque no quiere aprobar a un alumno que no cumple los requisitos para pasar de curso. En paro, se pone al frente de un negocio que lo acabará cambiando por completo. Poco a poco, va dejando de lado al honesto profesor para convertirse en un negociante sin escrúpulos al que sólo importa su propio beneficio.