Lord Windemere is schtupping Mrs. Erlynne, and everyone in London society knows it, with the exception of Lady Windemere.
These are the facts at the start of Oscar Wilde's play, "Lady Windemere's Fan." It is Lady Windemere's 21st birthday. Her husband has given her a lovely fan, and friends stop by to pay their respects.
One of them, the Duchess of Berwick, spills the beans that Lord Windemere is consorting with Mrs. Erlynne, and giving her huge sums of money. Lady Windemere finds this ridiculous.
Nonetheless, after the Duchess leaves, Lady Windemere goes through her husband's desk, and discovers--alas--that he has indeed been lavishing huge sums on Mrs. Erlynne.
Her husband comes in; they quarrel, and he insists on inviting Mrs Erlynne to his wife's birthday dance that very night.
This is one of Oscar Wilde's better-known plays, the first of his so-called "Society Comedies." In these, he skewers London "Society" and its foibles: class snobbery, materialism, etc. These plays, ironically, were hugely successful among the very people he mocked.
"Lady Windemere's Fan" is a very good play, funny in parts, without becoming bogged down with too much moralizing. By the play's end, Lord and Lady Windemere are happy again; Mrs Erlynne has been restored to society, and she's found love with a Lord of her own.
The "big secret" remains secret, and both playwright and audience live to grapple another day.