Act III

Lady Windermere is alone in Lord Darlington's rooms unsure if she has made the right decision. Eventually, she resolves to return to her husband, but then Mrs Erlynne appears. Despite Mrs Erlynne's honest attempts to persuade her to return home to her husband, Lady Windermere is convinced her appearance is part of some plot conceived by her and Lord Windermere. Mrs Erlynne finally breaks Lady Windermere's resistance by imploring her to return for the sake of her young child, but as they begin to exit they hear Lord Darlington entering with friends. The two women hide.

The men – who include Lord Windermere and Lord Augustus – have been evicted from their gentlemen's club at closing time and talk about women: mainly Mrs Erlynne. One of them takes notice of a fan lying on a table (Lady Windermere's) and presumes that Lord Darlington presently has a woman visiting. As Lord Windermere rises to leave, the fan is pointed out to him, which he instantly recognises as his wife's. He demands to know if Lord Darlington has her hidden somewhere. Lord Darlington refuses to co-operate, believing that Lady Windermere has come to him. Just as Lord Windermere is about to discover Lady Windermere's hiding place, Mrs Erlynne reveals herself instead, shocking all the men and allowing Lady Windermere to slip away unnoticed.

I am afraid I took your wife's fan in mistake for my own, when I was leaving your house to-night. I am so sorry.

—Mrs Erlynne

Act IV

The next day, Lady Windermere is lying on the couch of the morning room anxious about whether to tell her husband what actually happened, or whether Mrs Erlynne will have already betrayed her secret. Her husband enters. He is sympathetic towards her and they discuss the possibility of taking a holiday to forget the recent incident. Lady Windermere apologises for her previous suspicion of her husband and behaviour at the party, and Lord Windermere makes clear his new contempt for Mrs Erlynne – warning his wife to stay away from her.

Mrs Erlynne's arrival is announced along with the return of the fan, and despite her husband's protestations, Lady Windermere insists on seeing her. Mrs Erlynne enters and states that she shall be going abroad, but asks that Lady Windermere give her a photograph of herself and her son.

Whilst Lady Windermere leaves the room to find one, the story is revealed: Mrs Erlynne left her husband for a lover shortly after Lady Windermere's birth. When her new lover abandoned her, Mrs Erlynne was left alone and in disrepute. More recently, using the assumed name of Mrs Erlynne, she has begun blackmailing Lord Windermere to regain her lifestyle and status, by threatening to reveal her true identity as Lady Windermere's shameful mother — not dead, as Lady Windermere believes. Lord Windermere laments not having told his wife the whole story at once and resolves to tell her the truth now. Mrs Erlynne forbids him to do so, threatening to spread shame far and wide if he does.

Lady Windermere returns with the photograph which she presents to Mrs Erlynne, and requests that Lord Windermere check for the return of Mrs Erlynne's coach. Now that they are alone, and being owed a favour, Mrs Erlynne demands that Lady Windermere not reveal the truth about the events of the previous night to Lord Windermere, and Lady Windermere promises to keep the secret.

After Lord Windermere's return, Lord Augustus enters. He is shocked to see Mrs Erlynne after the events of the night before, but she requests his company as she heads to her carriage, and he soon returns to the Windermeres with news that she has satisfactorily explained the events of the evening, and that they are to marry and live out of England.

[Taking Lord Augustus' hand.] Ah, you're marrying a very good woman!—Lady Windermere

Their marriage is restored, but both Lord and Lady Windermere keep their secrets.