In a letter, the poor, recently widowed Lady Susan Vernon invites herself to stay at Churchill, the estate of her brother-in-law, Charles Vernon, and her sister-in-law, Catherine Vernon. She reveals in a different letter to her friend, Alicia Johnson, that this isn’t by choice—she was previously staying with a married man, Mr. Manwaring, and his wife, Mrs. Manwaring. While she was there, she seduced Mr. Manwaring and a man named Sir James Martin, intending to secure Sir James as a husband for her daughter, Frederica. However, Sir James was already engaged to Mr. Manwaring’s sister, Miss Manwaring. In the wake of the chaos she caused, Lady Susan had to leave, and she now plans to drop Frederica off at boarding school en route to Churchill. Meanwhile, corresponding with her mother Lady De Courcy and brother Reginald De Courcy, Catherine notes that Lady Susan has a bad reputation—only Reginald, hoping to be amused, is excited about her visit. Six years before, Lady Susan tried to prevent Charles from marrying Catherine, and Catherine hasn’t forgiven her.
When Lady Susan arrives, her behavior doesn’t match up with her reputation, and she’s able to charm Charles—though Catherine recognizes that Lady Susan has a talent for lying. In a letter to Alicia, Lady Susan reveals that she plans to force Frederica to marry Sir James against her will—boarding school is just a temporary punishment. Meanwhile, Reginald begins to change his opinion of Lady Susan, prolonging his stay to spend time with her; while Lady Susan has no plans to marry him and is still toying with Manwaring from afar, she enjoys tormenting Catherine by flirting with Reginald. Catherine, believing that Reginald might actually be in love with Lady Susan, shares her concerns with Lady De Courcy—but her letter finds its way to her father, Sir Reginald De Courcy. Sir Reginald writes to Reginald, disapproving of Lady Susan, both because she would make Reginald unhappy and because her reputation would damage the family. Reginald responds that he won’t marry her, but that her reputation is undeserved—Catherine, however, still believes that the match will happen one day.
Catherine also reveals that Frederica has run away from boarding school, and Frederica soon arrives at Churchill. By observing Lady Susan with Frederica, Catherine concludes that Lady Susan doesn’t love her daughter—but Reginald, still infatuated with Lady Susan, blames Frederica for everything. Meanwhile, Frederica grows enamored with Reginald, and Catherine approves of the match because it would separate Reginald from Lady Susan. However, Sir James soon arrives at Churchill, to the surprise of both the Vernons and Lady Susan. Frederica (who despises Sir James) is alarmed, but Lady Susan tries to convince Catherine that she planned it all along, and that Sir James and Frederica are happily engaged. Out of desperation, Frederica secretly gives Reginald a letter, in which she begs him to convince Lady Susan to break her engagement. Reginald attempts to do so and becomes disillusioned with Lady Susan because of her unkindness to Frederica.
Catherine then writes to her mother to tell her that Reginald plans to leave Churchill, which is a relief—not long after, however, Lady Susan convinces him to stay by pretending that she’ll leave instead. Lady Susan tells Alicia that she’s furious with Reginald and might marry him just to punish him; for now, however, she’s going to London to see Alicia. Alicia warns Lady Susan that Mr. Manwaring is still infatuated with her; if she wants to marry Reginald, she’ll have to get him out of the way. Furthermore, Alicia’s husband, Mr. Johnson, will be in town at the same time—he hates Lady Susan, so the two will have to avoid him in order to enact their schemes. Lady Susan, who’s begun spending time with Mr. Manwaring again, tries to convince Reginald—to whom she is now engaged—to stay away from London temporarily, claiming that she doesn’t want to upset his family with their engagement. In reality, she hopes to delay their marriage until after Sir Reginald is dead, because then the two will inherit Sir Reginald’s money and be financially independent.
Undeterred, Reginald comes to London anyway. His visit has disastrous consequences for Lady Susan, as he runs into the jealous Mrs. Manwaring at the Johnsons’ home—Mr. Johnson is her guardian, and she came to beg for his interference in Lady Susan’s affair with Mr. Manwaring. Reginald then learns all about the affair from Mr. Johnson, and he writes to Lady Susan to break their engagement. This time, Lady Susan is unable to change his mind. Alicia advises Lady Susan to marry Sir James instead and tells her that, due to Mr. Johnson’s interference, she can no longer correspond with Lady Susan.
An unnamed narrator reveals that, while Lady Susan initially persisted in forcing a marriage between Frederica and Sir James, she eventually changed her mind and returned Frederica to Churchill. Lady Susan then married Sir James herself, and after a year, Reginald’s family persuaded him into loving and marrying Frederica. Ultimately, the narrator does not pity Lady Susan and instead pities Sir James and Miss Manwaring, who lost Sir James to Lady Susan.
