Inside the country house, Gwendolen and Cecily look out of the window into the garden. Jack and Algernon enter. After asking the men to explain themselves, the women decide to forgive them, then quickly change their minds. Their "Christian names are still an insuperable barrier." However, the men reveal that they are to be re-christened this afternoon, and the couples hug and make up. Lady Bracknell arrives, and Gwendolen informs her of her engagement. Lady Bracknell tells Jack that he may not speak any more to her daughter. Lady Bracknell asks Algernon about his friend Bunbury; he says that Bunbury died that afternoon.
Jack introduces Cecily to Lady Bracknell, and Algernon says that he is engaged to her. To Jack's increasing frustration, Lady Bracknell continually doubts the respectability of Cecily's background. Only when Lady Bracknell discovers Cecily has a large personal fortune does she warm to her and give her consent to the match. Jack, however, says that as his ward, Cecily may not marry without his consent, and he declines to give it. He says that he suspects Algernon of being untruthful. He recounts this afternoon's events, in which Algernon impersonated Jack's brother. He reveals that Cecily is under his guardianship until she turns 35. Cecily feels she cannot wait this long to be married. Jack tells Lady Bracknell that if she consents to his marriage with Gwendolen, he will consent to Cecily's with Algernon. Lady Bracknell refuses and tells Gwendolen to get ready for the train.
Chasuble enters and announces that he is ready to perform the christenings. Lady Bracknell refuses to allow Algernon to be baptized, and Jack tells Chasuble that the christenings will not be necessary any more. Chasuble says he will leave, and mentions that Miss Prism is waiting for him. Lady Bracknell knows of Miss Prism and says she needs to meet her. Miss Prism enters and, upon seeing Lady Bracknell, goes pale. Lady Bracknell accuses her of kidnapping a baby boy from her house 28 years ago. Under Jack's questioning, Miss Prism reveals she accidentally left the baby in a handbag on the Brighton railway line. Jack leaves excitedly.
Jack returns with the handbag. Miss Prism recognizes it as her own. Jack tells her that he was the baby. Lady Bracknell informs Jack that he is the son of her sister, making him Algernon's older brother. Jack asks Lady Bracknell what his original name was. She says he was named after his father, but she cannot remember his name, nor can Algernon. They locate his name under the Army Lists, as he was a General: Ernest John Moncrieff. Gwendolen is ecstatic. All three couples, Chasuble and Miss Prism, Algernon and Cecily, and Jack and Gwendolen, embrace. Jack tells Lady Bracknell that he has realized, for the first time in his life, "the vital Importance of Being Earnest."
Wilde's ironies explode in the whirlwind last-minute revelations. As Jack (or Ernest) says, "it is a terrible thing for a man to find out suddenly that all his life he has been speaking nothing but the truth." The various conflicts from lies are resolved since they turn out (for the most part) not to be lies after all. Jack truly is Ernest, Algernon truly is his mischievous brother.
But if the lies in the play are true, then what can we make of the supposed truths? On the flip side, we see that so many of these truths--especially social truths--are lies. For instance, Lady Bracknell's sudden liking to Cecily, after hearing of her personal fortune, is a "lie"; she puts on her "shallow mask of manners" to hide her obvious materialism. Wilde's most concrete attack throughout the play is on marriage as a social tool, and he provides even more absurd obstacles in the final act: Jack holds Cecily under his guardianship until she is 35, Gwendolen still refuses to marry Jack until she has proof his real name is Ernest. These example prove how absurd class as a more conventional obstacle to marriage is.
Wilde's structural craftsmanship emerges. What was previously a throwaway joke--that the railway line Jack was abandoned on was "immaterial," as Lady Bracknell dismissed--turns out to be crucial information in Jack's realization of his origins. Tight dramatic structure like this allows the audience to forgive the even sillier coincidence that Jack happens to have the Army Lists at hand (made funnier by his explanation that "These delightful records should have been my constant study"). Even Chasuble and Miss Prism's union at the end delights us; Wilde has portrayed them in enough serious light as a perfect match that we ignore their over-the-top embrace.
It is also worthwhile to note Wilde's continued use of rapid contrasts and shifts. Gwendolen and Cecily change their minds repeatedly at the start of the act, vowing not to speak to the men before immediately doing just that. This is summed up no better than when Gwendolen asks Cecily if they should forgive them; Cecily replies "Yes. I mean, no." Wilde is not merely using these reversals for humor; he shows how absurd romantic decisions of the heart become when entwined with even more absurd social conventions.