Act 3, Scene 1. The North Parade. Jack has learned that the girl his father wants him to marry is "the very girl [he is] plotting to run away with." When his father, Anthony, comes in, Jack tells him that he has thought about it and he is willing to marry the woman his father has chosen.
Anthony is pleased to hear it, and tells Jack that he is marrying him off to Lydia Languish. Playing with his father, Jack pretends to be indifferent to Lydia, even though Anthony was excited to present such an eligible, young, and beautiful woman to his son. "What think you of blooming, love-breathing seventeen?" Anthony squawks, determined to convince Jack that he has found a good match.
When Jack remains indifferent, Anthony says, "Why, you unfeeling, insensible puppy, I despise you! When I was of your age, such a description would have made me fly like a rocket!" Frustrated, Anthony becomes determined to send Jack to Lydia at once.
Scene 2. In Julia's dressing room, Faulkland is alone. He remembers when he and Julia first fell in love, when suddenly Julia enters. She confronts Faulkland about the fact that he has been cold, and that he is often doubtful of her love for him. He confronts her about her actions, and suggests that she does not love him, but she insists she does. When he doesn't believe her, she flees the room, weeping. Seeing how upset she is, Faulkland expresses regret for his behavior, and vows to himself to make it up to her the next time he sees her.
Scene 3. At her lodgings, Mrs. Malaprop speaks to Jack Absolute about his imminent marriage to Lydia. Jack flatters Malaprop, citing her "intellectual accomplishments, elegant manners, and unaffected learning." After he flatters her a great deal, Malaprop tells Jack about the fact that Lydia has been corresponding with another man, pulling out a note from "Ensign Beverley." "Oh, the devil! my last note," says Jack in an aside, annoyed that Lucy would hand the note over to Malaprop.
Jack reads the letter aloud, which refers rather unflatteringly to Malaprop, calling her an "old weather-beaten she-dragon." When Jack reads a part that insults Malaprop's poor use of language, Malaprop gets very angry, and Jack pretends to be on her side, saying, "He deserves to be hanged and quartered!"
Jack hatches a plan to let Lydia keep corresponding with her lover, and even let her plan an elopement, which they will then block at a later date. Malaprop is delighted by this plan. Jack then asks to speak with Lydia for a moment, advising Malaprop to tell Lydia that Beverley is there to see her; he poses this idea as a kind of joke that Malaprop can play on Lydia.
Lydia enters, thinking she is about to meet Jack Absolute, but is surprised when the man she meets is the man she knows to be Ensign Beverley. Jack keeps up the charade and tells her, "I have deceived your aunt—I was informed that my new rival was to visit here this evening, and contriving to have him kept away, have passed myself on her for Captain Absolute." Lydia is delighted to hear that Malaprop thinks he is Jack Absolute.
Lydia asks "Ensign Beverley" if he is ready to marry her and forfeit her inheritance for the sake of romance. When he says yes, she beams, saying, "how charming poverty will be with him!" Malaprop comes in and listens in on their conversation. She hears Lydia say, "Let her choice be Captain Absolute, but Beverley is mine," and is surprised that Lydia would say this to Captain Absolute's face, unaware that Lydia thinks he is Beverley. Malaprop confronts Lydia about insulting Captain Absolute to her face, but everyone remains blissfully unaware of the fact that Jack and Beverly are the same person.
Scene 4. Acres' lodgings. Acres is there with David, his servant. They are discussing the fact that Acres has recently changed his wardrobe to be more sophisticated and urbane, after years of country living. Acres then talks about how he likes to dance in a country style, when they are interrupted by the arrival of Lucius O'Trigger.
Lucius comes in and greets Acres, asking what has brought him to Bath. Acres tells him that he is in love with a woman, without mentioning her name. He tells Lucius that he has a rival named Beverley, who is also in love with the woman. Lucius encourages Acres to fight Beverley, but Acres insists, "...he has given me no provocation." Lucius insists that the fact that they love the same woman is provocation enough, and begins to stoke some animosity in Acres, who becomes determined to challenge Beverley to a duel. He becomes impassioned and sits down to write a letter. In the letter, he summons Beverley to King's-Mead-Fields. Before he leaves, Lucius tells Acres that he has a rival he would like to fight as well.
So much of the pleasure of the play comes from dramatic irony, the fact that the audience is privy to all the intricacies of the plot, while many of the characters are blissfully unaware. The pleasure of dramatic irony is exploited at the beginning of the act, when Jack has learned of his father's plot to marry him to the woman he already loves. While the audience knows that Jack loves Lydia and is more than willing to marry her, Jack pretends to be indifferent to the match, which infuriates his father. This tension, between what the audience knows and what Anthony knows, creates an especially comedic scenario.
The secondary couple in the play, Julia and Faulkland, is not as intricately entangled as Jack and Lydia, but they create their own complications in the dynamic they have struck up. Faulkland is exceedingly insecure, and interprets nearly everything that Julia does as proof that she does not love him. No matter what Julia says, he cannot be consoled; he even sees her happiness in his absence as evidence of her inconstancy. While they have both expressed their affection for one another, Faulkland is unable to accept it, and sees Julia as disloyal, even though she is not.
Matters only get more and more complicated as the play progresses. Jack plots to win over Lydia, maintaining one identity with Malaprop and his father, and another with his lover, Lydia. He must keep his two identities straight, as a betrayal of his plot to either party will ruin his plans to marry the woman he loves. Meanwhile, the audience delights in his trickery, especially when it leads him into particularly contradictory territory, such as when he must read an insulting letter that he wrote as Beverley about Malaprop to Malaprop.
Through the character of Lydia Languish, the play satirizes a youthful orientation towards romance. Not only is her voracious consumption of romance novels comedic in its own right, but also her love for "Ensign Beverley." So much of her affection for him comes from the fact that he tells her he is willing to give up earthly goods for the sake of love. His lack of materialism, and his willingness to "forfeit that portion of [her] paltry wealth" is what makes him so appealing, as it seems to prize affection and love over financial security. To a 17-year-old rich girl, there is nothing more romantic than being poor, and some of Lydia's most comic moments come from her preoccupation with penniless love, as when she says, "How charming will poverty be with him!" The joke for the audience is, of course, that poverty is never charming, and Lydia is naive to believe that it could be.
While Jack Absolute's plot has been mostly smooth throughout the play, he faces some complication when Lucius O'Trigger encourages Bob Acres to challenge "Beverley" (Jack) to a duel. While he has imagined that he will be able to win Lydia's hand in marriage by posing as an ensign, his love is not unrivaled. The impressionable Bob Acres takes Lucius' call to arms seriously and becomes impassioned in his quest to protect his affection for Lydia. The act finishes with the promise of violence and retribution for our hero, Jack Absolute.