The old bachelor Don Pasquale anxiously awaits the arrival of his friend, Dr. Malatesta. Even at his somewhat advanced age, Pasquale has decided to marry, mostly to punish his rebellious nephew, Ernesto. It seems that Ernesto is in love with Norina, a beautiful young widow. He refuses to marry the woman of his uncle’s choice. By marrying, Don Pasquale will provide himself with an heir, thereby leaving Ernesto penniless. Malatesta has promised to suggest a prospective bride for Pasquale. But the old man does not know that he means to help the young couple and plots to put Pasquale at their mercy.

Malatesta arrives full of praise for his younger sister. Her convent upbringing, he claims, has taught her the value of frugality and modesty. She is a perfect bride for the Don! As Malatesta leaves, he tells the already infatuated Pasquale that he will bring his sister for inspection that very evening.

When Ernesto arrives and, again, refuses to marry anyone but Norina, Pasquale orders him out of the house. He disinherits him and announces his own wedding plans. The astonished Ernesto is even more shocked to learn that his friend, Dr. Malatesta, has arranged the marriage.

Norina is reading a book on courtly love. She receives a note from Ernesto, bidding her farewell. Pasquale will not allow them to marry, and Malatesta has betrayed them. Malatesta enters and cheers her up by letting her in on his plan: she will play the part of his simpleminded, modest sister, “Sofronia.” Once she and the old man have had a mock wedding, she will drive him crazy until he begs to be rid of her. Intent on their plan, Malatesta and Norina rehearse the scene, then leave together.

In Pasquale’s house, a lamenting Ernesto, ignorant of Malatesta’s plan, packs his bags. He departs and Pasquale enters, eagerly anticipating his meeting with his bride-to-be. The Doctor arrives with “Sofronia,” who pretends to be unwilling. Delighted, Pasquale sends, immediately for a notary to draw up the marriage contract. Malatesta ushers in a cousin, whom he has enlisted to perform the mock ceremony. But a second witness is required. Ernesto bursts in, and Malatesta quickly lets him in on the hoax and recruits him as a witness.

No sooner is the ceremony over than “Sofronia” becomes a monster. She tells Pasquale he’s too old to escort her in public. From now on, Ernesto will do the honors. What’s more, if she is expected to live in Pasquale’s house, she will have to have additional servants, and the whole place will have to be redecorated. Norina and Ernesto laugh together, and Malatesta tries to calm the outraged Pasquale.

An army of new servants has been employed and are at work, redecorating as the bewildered old man looks on. As he examines the mountain of bills before him, “Sofronia” announces that she is going to the theater. When Pasquale forbids her to leave on their first evening of marriage, she slaps him. In disbelief and anger, he shouts, “Divorzio, divorzio.” Feeling a little guilty, she tries to charm him again. Soon, however, she turns to leave for the theater and, intentionally, drops a note. Pasquale retrieves it once she is gone. It is from a lover, arranging a rendezvous in the garden that evening. Appalled, the Don goes in search of Malatesta. The Doctor has just instructed Ernesto to meet him in the garden, later, but pretends to sympathize with Pasquale about the slap and the note. Together, he asserts, they will trap the guilty pair that very evening.

In the garden, Ernesto serenades Norina. Knowing they are being observed, they throw themselves into a passionate duet. When Pasquale rushes forward to denounce them, Malatesta finally reveals the hoax. Pasquale is torn between anger and relief. But he relents and blesses the pair. All join in the final chorus: “An old man who marries can only expect trouble!”