Young Captain Absolute, son and heir of Sir Anthony Absolute, arrives in Bath to pay court to the rich and lovely Lydia Languish. His suit is singularly complicated because he has made himself known to her as the penniless Ensign Beverley, the better to intrigue her romantic nature. Lydia, seventeen, favors the excitement of an elopement, but Captain Absolute is aware that she will lose two-thirds of her fortune if she weds without the consent of her aunt, Mrs. Malaprop. He hopes that Lydia will accept him in his true name after she has come to love him as Ensign Beverley.
Lydia also has problems: her aunt has intercepted a note from Beverley and has confined Lydia to her home; now she has no opportunity to patch up a petty quarrel with her lover, and fears that she has lost him. Her friend Julia tries to console her by saying that, after all, Beverley is penniless, but Lydia declares herself determined to marry, before she becomes of age, a man who will care nothing for her fortune.
She tells Julia that Mrs. Malaprop has not scrupled to carry on a small romance of her own: she is corresponding, under the name of Delia, with a fire-eating Irish baronet, Sir Lucius O'Trigger, who is unaware of her true identity. Mrs. Malaprop's shrewd maid, Lucy, who acts as messenger between them, is fattening her purse by telling the impoverished O'Trigger that "Delia" is the beautiful Lydia.
A new complication now arises: Sir Anthony makes a suprise visit to Bath. He arrives with Mrs. Malaprop at her home to propose a match between his son, the Captain, and Lydia. Mrs. Malaprop, who has an amazing propensity for garbling the English language, orders Lydia to "illiterate" Beverley from her thoughts. But Lydia, unaware of his true identity, refuses to marry Captain Absolute. In spite of her refusal, her aunt accepts his father's proposal, and prepares to dismiss another of Lydia's suitors, Bob Acres, a young man who is somewhat of a bumpkin.
Captain Absolute has learned of the arrival of his father and Julia, who is Sir Anthony's ward, and he summons his friend Faulkland to give him the news about them. Faulkland and Julia are betrothed, but the former is in a perpetual stew of doubts, fears, hopes and wishes, all revolving around his beloved. For amusement, the Captain calls in Bob Acres to report on Julia's health (he is a country neighbor of the Absolutes), and to hear Acres berate his rival, Beverley, not knowing that the latter is, in reality, his friend Absolute. Faulkland, who has been worrying for fear Julia might be ill, hears that she is quite merry in spite of his absence, and is thrown into a new fever of unhappiness.
The testy Sir Anthony calls in order to command his son to marry Lydia, but the Captain refuses--his father neglects to tell her name--and Sir Anthony stamps out, threatening to disinherit him. Fag, the Captain's servant, learns from Lucy that Sir Anthony's choice is Lydia, and this he tells young Absolute. The enlightened Captain hastens to his father to say that he has repented and is willing to court Lydia. Father and son set off to pay their addresses to Mrs. Malaprop.
This lady, after approving the Captain as "the very pineapple of politeness," tells them that she has intercepted another note from Beverley to Lydia--in which, unfortunately, he refers to Mrs. Malaprop as "a weather-beaten she-dragon." The letter also reveals that Beverley has a scheme to see Lydia--with "the old harridan" as an unwitting go-between.
Young Absolute suggests that Mrs. Malaprop punish the conceited puppy, Beverley, by letting him reach the point of elopement; then he, Absolute, will himself carry off Lydia. She agrees, and Lydia is summoned. "Beverley" whispers to her that he has disguised himself as Absolute, and the delighted Lydia tells her aunt again that she will wed only Beverley. Mrs. Malaprop declares Lydia to be as headstrong as "an allegory on the banks of the Nile."
Meanwhile, Acres, rebuffed by Lydia and blaming the mysterious Beverley for her coldness, is urged by O'Trigger to challenge his rival to a duel. A note is written to Beverley, naming that very evening for the duel in King's Mead-Fields. O'Trigger himself sets out in search of Captain Absolute (whom he believes to be his rival for "Delia") with the idea in mind of challenging Absolute to a duel. Acres, in preparation for his tilt with Beverley, asks young Absolute to be his second. The waggish Captain declares that he thinks this hardly proper, but he agrees to deliver Acres note to Beverley.
Sir Anthony Absolute now insists on taking his son to Lydia's home. Here he acknowledges him in her presence, and Lydia at once realizes that there has been a hoax--Beverley, of course, is really the Captain. Mrs. Malaprop agrees to forgive all, and says: "We will not anticipate the past, our retrospection will now be all to the future"; but Lydia, angry at being duped, declares that indeed she renounces "Beverley" forever, and flounces from the room.
The Captain, infuriated by Lydia's behavior, leaves at once. He meets O'Trigger who is seeking to challenge him, they quarrel and agree to cross swords that evening in the King's Mead-Fields--where Acres is scheduled to meet Beverley. Absolute informs his friend Faulkland of the coming event, giving the latter a new idea for testing Julia's love for him: he tells her that he has involved himself in a quarrel and must run away immediately. Julia is ready to accompany him, but, learning that the story is another one of Faulkland's concoctions, declares that now she will never marry him.
Lydia, Julia and Mrs. Malaprop hear from the servants a confused story of the impending duel--a duel in which Absolute, Faulkland and O'Trigger are named as the principals--and they hasten to the field to prevent what Mrs. Malaprop fears is to be "fine suicide, paracide, salvation and an antistrophe." Sir Anthony, who has met his son on his way to keep his engagement but who has been deceived as to the purpose of young Absolute's sword, now learns of the impending duel, and sets out for the King's Mead-Fields.
Here, the bloodthirsty O'Trigger is giving Acres some preliminary instructions in duelling, but so graphically does he illustrate the lesson that Acres has quite lost his appetite for combat. Young Absolute and Faulkland arrive. O'Trigger, learning that Faulkland is not Beverley (he has assumed from the beginning that he was), proposes that Faulkland fight Acres anyway, just to make a foursome. Acres hurriedly vetoes that suggestion. Absolute then identifies himself as Beverley. Acres, much to O'Trigger's disgust, now insists that he cannot possibly fight his friend Absolute. Absolute and O'Trigger are drawing their swords when Sir Anthony and the women appear.
O'Trigger greats Lydia as his "Delia," and is unpleasantly surprised to learn that his correspondent has been, in reality, the simpering Mrs. Malaprop. He promptly relinquishes "Delia" to Absolute. Lydia forgives the Captain, and he and O'Trigger are quickly reconciled. Faulkland and Julia also grant forgiveness to each other, and plan to be married at once. Bob Acres, vastly relieved, renounces all claims to any wife for whom he must fight, and invites the company to a party.