7 months prior to the writing of the novel's first letter, 35-year-old Lady Susan Vernon's husband died. She spent the last three of those months with the Mainwarings, but she finds herself no longer welcome after starting an affair with Mr. Mainwaring and ruining the engagement between their niece Miss Mainwaring and the wealthy but slow-witted Sir James Martin. Her goal, however, isn't to marry Sir James herself but to have him marry her 16-year-old daughter Frederica. Although the conflict revolves around her greatest enemy — her mother — instead of her, Frederica breaks crucial rules of etiquette (particularly regarding interaction between men and women) like Marianne Dashwood, dares to show attraction to a man before he shows attraction to her like Catherine Moreland, is persecuted by a Wicked Stepmother-esque guardian like Fanny Price, is the Black Sheep of her family and criticized for her quietness and self-control like Elinor Dashwood and Anne Elliot, and is no dashing beauty (like her mother) who effortlessly charms men at first sight but is misjudged and dismissed as insignificant and undesirable in every way by her future husband until he learns the error of his first impressions like Elizabeth Bennet was, and most importantly, is determined to Marry for Love like any Jane Austen heroine.
As punishment for refusing to marry Sir James, Lady Susan sends her daughter to Miss Summers' Boarding School until she changes her mind. As she is not patient enough to wait for Mrs. Mainwaring to die and free Mr. Mainwaring to marry her, Lady Susan removes herself to her husband's brother's estate of Churchhill, to the extreme displeasure of his wife Catherine, who, unlike her husband Charles, has not forgotten how Lady Susan tried to sabotage their marriage and prevented Charles from purchasing the family estate Vernon Castle from his brother. While at Churchhill, Lady Susan forms a second goal — to seduce Catherine's brother Reginald de Courcy. In a matter of weeks, Lady Susan has him completely duped and wrapped around her finger. She doesn't find him nearly as satisfying as the slavish, worshipful Mr. Mainwaring and isn't terribly excited about marrying him unless his father dies, increasing his fortune, but conquering his initial dislike of her is just too much fun!
Lady Susan eventually puts her first plan into action and writes to Frederica of her pending Arranged Marriage to Sir James. There's only one thing a girl can do in this situation — run away. Miss Summers refuses to readmit the girl into her school, so the Vernons bring her to stay with them. Catherine soon notices, to her delight, that her niece is falling in love with Reginald and hopes that she could be the means of freeing him from his blind devotion to Lady Susan. When Sir James shows up and the truth about Lady Susan's intentions, her reprehensible treatment of her daughter, and Frederica's abhorrence for the marriage come out, it looks like Reginald's eyes will be opened at last. Alas, Lady Susan is able to smooth things over with Reginald — which throughly pleases her — but at the cost of getting rid of Sir James and temporarily setting aside her first Evil Plan, which does not please her. Her best friend and sidekick-of-sorts, Alicia Johnson (whose husband was Mrs. Mainwaring's guardian), advises Lady Susan to leave Churchhill at the same time Reginald does, eventually marry him, come to London and either renew or finally end her affair with Mainwaring, and leave Frederica behind with the Vernons where she won't bother her. Lady Susan follows her friend's plan.
