Don Pasquale da Corneto, a rich, eccentric bachelor in his sixties, has taken it into his head to disinherit his nephew Ernesto for refusing to marry according to his uncle's wishes. His friend Dr. Malatesta reports on a lovely young woman he later identifies as her sister from the convent, whom Pasquale himself would like to marry. With increasing enthusiasm, Pasquale hopes to start a family and teach his nephew a lesson. When Ernesto arrives, he asks him once again to do hissbidding. Ernesto replies he cannot marry anyone else but a certain Korina, whom Pasquale dismisses sight unseen, because she has no dowry. Then he declares that he is realizing his threat to disinherit Ernesto - he himself is going to take a wife! When Ernesto advises consulting his friend Malatesta, Pasquale retorts that the intended bride is Malatesta's own sister. Ernesto feels betrayed and hopeless, knowing the latter is none other than Korina.
Korina is reading a knightly romance at home. When her brother enters she disclaims her part in the joke they are planning to play on Pasquale, giving Ernesto's despair as her reason. Malatesta promises her to reveal their secret to Ernesto. All she has to do, he says, is to drive Pasquale crazy once the bogus marriage contract is signed.
Not yet having learnt of his friends' plot, Ernesto intends to go into exile. His uncle appears in his finest suit, impatiently awaiting Malatesta, who soon arrives with the modest "Sofronia". Pasquale pictures her as retiring and domestic. Malatesta summons the waiting "Notary" (his cousin Carlotto in disguise) to produce a contract. Malatesta dictates the terms, making it clear that half of the Don's property will go to his wife. Ernesto bursts in, but Malatesta pulls him aside and explains that the plot is for his benefit. Ernesto begins to catch on when Korina starts to behave shrewishly towards her new "husband". Without further ado, she declares she wants no disobedience or trouble from Pasquale, then orders extra servants, a new carriage, furnishings, clothes and jewellery. Pasquale recognizes the extent of his mistake.
Later that evening, Don Pasquale sits facing a stack of bills, realizing he will soon be ruined at this rate. Korina is getting ready for an evening at the theatre and he accosts her, but she slaps him when he orders her to stay at home. He declares he will get a divorce. On her way out, she accidentally drops a note from a lover, setting a rendezvous in the garden that night. Don Pasquale plots with Malatesta to use the compromising situation to get rid of "Sofronia". Ernesto meets Korina in the garden. They declare their love. When they hear Pasquale and Malatesta approaching, Ernesto ducks into the house while Pasquale confronts his "wife", who denies she has a lover and refuses to leave. Malatesta now plays his role, announcing to his sister that she ought to leave, because Ernesto's bride Korina will be there the next day. Pasquale welcomes the idea of Ernesto's marriage when he sees how "Sofronia" acts against sharing the house with another woman. He is only too glad to settle an income on his nephew. He is flabbergasted, of course, to learn that "Sofronia" is in fact Korina, but finally he gives the young couple his blessing and agrees to act his age.