The play is set in Bath in the 18th century, a town that was legendary for conspicuous consumption and fashion at the time. Wealthy, fashionable people went there to "take the waters", which were believed to have healing properties. Bath was much less exclusive than London, and hence it provides an ideal setting for the characters.
The plot centres on the two young lovers, Lydia and Jack. Lydia, who reads a lot of popular novels of the time, wants a purely romantic love affair. To court her, Jack pretends to be "Ensign Beverley", a poor officer. Lydia is enthralled with the idea of eloping with a poor soldier in spite of the objections of her guardian, Mrs. Malaprop, a moralistic widow. Mrs. Malaprop is the chief comic figure of the play, thanks to her continual misuse of words that sound like the words she intends to use, but mean something completely different. (The term malapropism was coined in reference to the character.)
Lydia has two other suitors: Bob Acres (a somewhat buffoonish country gentleman), and Sir Lucius O'Trigger, an impoverished and combative Irish gentleman. Sir Lucius pays Lucy to carry love notes between him and Lydia (who uses the name "Delia"), but Lucy is swindling him: "Delia" is actually Mrs. Malaprop.
As the play opens, Sir Anthony arrives suddenly in Bath. He has arranged a marriage for Jack, but Jack demurs, saying he is in love already. They quarrel violently. But Jack soon learns through the gossip of Lucy and Fag that the marriage arranged by Sir Anthony is, in fact, with Lydia. He makes a great show of submission to his father, and is presented to Lydia with Mrs. Malaprop's blessing. Jack confides to Lydia that he is only posing as Sir Anthony's son. She annoys Mrs. Malaprop by loudly professing her eternal devotion to "Beverley" while rejecting "Jack Absolute".
Jack's friend Faulkland is in love with Julia, but he suffers from jealous suspicion. He is constantly fretting himself about her fidelity. Faulkland and Julia quarrel foolishly, making elaborate and high-flown speeches about true love that satirise the romantic dramas of the period.
Bob Acres tells Sir Lucius that another man ("Beverley") is courting the lady of Acres' choice (Lydia, though Sir Lucius does not know this). Sir Lucius immediately declares that Acres must challenge "Beverley" to a duel and kill him. Acres goes along, and writes out a challenge note   despite his own rather more pacifist feelings, and the profound misgivings of his servant David. Sir Lucius leaves, Jack arrives, and Acres tells him of his intent. Jack agrees to deliver the note to "Beverley", but declines to be Acres' second.
Mrs. Malaprop again presents Jack to Lydia, but this time with Sir Anthony present, exposing Jack's pose as "Beverley". Lydia is enraged by the puncturing of her romantic dreams, and spurns Jack contemptuously.
Sir Lucius has also learned of the proposed marriage of Jack and Lydia, and determines to challenge Jack. He meets Jack, who, smarting from Lydia's rejection, agrees to fight him without even knowing the reason. They will meet at the same time as Acres is scheduled to fight "Beverley".
At the duelling ground, Acres is very reluctant to fight, but Sir Lucius will have no shirking. Jack and Faulkland arrive. Acres learns that "Beverley" is actually his friend Jack, and begs off from their duel. However, Jack is quite willing to fight Sir Lucius, and they cross swords.
David informs Mrs. Malaprop, Lydia, Julia, and Sir Anthony of the dueling, and they all rush off to stop it. Sir Lucius explains the cause of his challenge, but Lydia denies any connection to him, and admits her love for Jack. Mrs. Malaprop announces that she is Delia, but Sir Lucius recoils in horror, realising that he has been hoaxed. Sir Anthony consoles Mrs. Malaprop, Julia is reconciled to Faulkland, and Acres invites everyone to a party.
Output: Why does Lydia become upset with Jack when they are introduced with Sir Anothony in the room?