The son of Pantalone, the honest Venetian merchant, Lelio Bisognosi, the liar, is a brilliant young man wandering the world. For twenty years he lived with an uncle in Naples, then returned to his native city with his servant Arlecchino. Here, he has the opportunity to meet Dr. Balanzoni's daughters, Rosaura and Beatrice, while, in the absence of their father, they enjoy being serenaded by an unknown admirer on the terrace of the house. The author of the serenade is Florindo, a student and tenant of the doctor, who shyly loves Rosaura, not daring to reveal his feelings. Lelio does not miss the opportunity: he introduces himself to the girls in the guise of a rich marquis and claims that he is the author of the serenade, without revealing which of the two he is in love with. But for the sake of the art of "spiritual inventions", Lelio delights in describing himself in various lies, not without undesirable consequences...