The opera takes place in Italy, sometime during the nineteenth century.

Lattanzio Lattrughelli's opera company is in the process of rehearsing a new work, with the usual mishaps. His wife's niece, Eugenia, is in love with a young cavalry lieutenant, Edoardo, who loves her back. But his uncle, Colonel Franceschetti, refuses to consent to the marriage of his nephew with an actress. Aurelia, Eugenia's aunt and Lattanzio's wife, pretends to be a countess, and in the "happy deception" of the title, deceives the Colonel into believing she loves him, and in so doing persuades him to accept the union of the two young people. When he finds out that he has been tricked, the Colonel reacts angrily before agreeing to their wedding, and all ends happily.