Coscoletto is ordered to go every morning and every night to serenade Mariana, the beautiful wife of the sleepy macaroni dealer Frangipani. His wealthy master is Polycarpo, the owner of a cat-gut string factory. Polycarpo does not notice, however, that Coscoletto, following his own designs, directs his ardent songs not to Mariana (though she believes and hopes that he is doing so) but to her neighbour, the charming florist Delfina. So he combines profit with the pursuit of his goal and at the same time has an alibi against the suspicious Frangipani. However, when a love letter of Polycarpo, hidden in a bunch of flowers, reaches Delfina instead of Mariana, the entanglements begin. For Delfina, unable to read or write, asks Mariana of all people to help her write an answer. By means of a trick Mariana finds out that Polycarpo, not Coscoletto, has written the letter, and it is she who writes the answer on Delfine’s behalf. Delfine still believes that Coscoletto has written to her. Frangipani is furious with jealousy when, unnoticed, he witnesses Polycarpo read Mariana’s answer aloud to Coscoletto. In her letter, she rouses the receiver’s hopes, insisting on an immediate wedding. She writes that all possible obstructions could be easily removed. Frangipani suspects that he is going to be got rid of, and the strange behaviour of his friend Arsenico, a pharmacist (and poisoner) by trade, is not the only thing that gives him reason for unease. Vesuvius is rumbling and snarling, and Frangipani prepares for the attack...