Mrs. Mainwaring was an heiress with a large fortune who could have married a lord or a baronet, but instead, against the advice of her guardian Mr. Johnson, she married Mr. Mainwaring, a charming and handsome man with no money. She lives in Langford with her husband and his sister Maria. According to Alicia Johnson, she is thin and ugly and frets herself "thinner and uglier" over her philandering husband.
Lady Susan Vernon visits them at Langford after her husband's death. Mrs. Mainwaring becomes jealous of her husband's flirtation with their guest, which forces Lady Susan to leave for a visit to her brother-in-law Charles Vernon and his family at Churchhill. This does not stop Lady Susan and Mr. Mainwaring's relationship from developing further, however, and Mrs. Mainwaring goes to Mr. Johnson for help.
On one such visit, she runs into Lady Susan's suitor Reginald De Courcy and she informs him the rumors he had heard about Lady Susan's behavior at Langford were true and that Mr. Mainwaring was secretly visiting Lady Susan in London, causing a break between them. Angry at her interference and still interested in Mr. Mainwaring, Lady Susan hopes Alicia Johnson will continue to aggravate Mrs. Mainwaring's nerves and drive her to an early grave.