The plot unfolds in London over twenty-four hours, centering on the birthday celebration of young, puritanical Lady Windermere. Her complacent domestic life is abruptly shattered when malicious society gossips, led by the Duchess of Berwick, inform her that her husband, Arthur, is infatuated with a notorious woman named Mrs. Erlynne, regularly visiting her and providing her with large sums of money. Driven by distress and jealousy, Lady Windermere intercepts her husband's private bank book and discovers the damning financial records, confirming the affair to her horror.
Despite his wife's anger and explicit refusals, Lord Windermere insists on inviting Mrs. Erlynne to her birthday ball, claiming it is her last chance to re-enter respectable society. Outraged and feeling utterly degraded, Lady Windermere resolves to leave her husband. When Lord Darlington seizes the moment to declare his passionate love and urges her to abandon her home, she flees in desperation to his bachelor rooms, leaving a farewell letter behind for her husband.
Meanwhile, Mrs. Erlynne discovers the incriminating letter at Lord Windermere's house and realizes her own tragic history is repeating itself through her daughter. Driven by a sudden, fierce maternal instinct, she follows Lady Windermere to Lord Darlington's apartments to save her from ruin. When the men unexpectedly arrive from the club, Mrs. Erlynne hides Lady Windermere behind a window curtain and ultimately sacrifices her own reputation by stepping forward to claim the abandoned fan as her own, taking the blame to prevent public scandal.
The following morning, Mrs. Erlynne visits the Windermere residence to return the fan and bid a final farewell before leaving England. She extracts a solemn promise from Lady Windermere to keep the events of the previous night a secret, thereby preserving the young couple's marriage and protecting her daughter's illusions. As she prepares to depart, Lord Augustus Lorton announces that Mrs. Erlynne has accepted his marriage proposal, and the couple resolves to live abroad, leaving behind a society fundamentally blind to the true depths of human sacrifice.
