The scene takes place in the garden of Jack Worthing’s country manor house. Miss Prism is calling to Cecily to do her schoolwork. Cecily dislikes learning German, which her uncle seems to consider important. Cecily worries about her uncle’s health because he is so serious, but Miss Prism says he is robust and very responsible for his age. She believes he tends to be worried about his brother Ernest—who Cecily desires to meet. Miss Prism doesn’t believe in reforming bad characters. People should pay the consequences for their actions.
Dr. Chasuble enters. Cecily tries to encourage him to take Miss Prism for a walk. He asks when Jack is due back, and Miss Prism replies that he’ll return on Monday. Everyone considers Jack to be a severe, solemn man. The reverend seems to be attracted to Miss Prism. She decides she will walk with him. She assigns Cecily her schoolwork, who hates it.
Merriman enters. He tells Cecily that Jack’s brother Ernest has arrived. Ernest was disappointed when told that Jack wasn't there but wishes to speak to Cecily. She orders a room prepared for him. She is frightened of meeting a wicked person, fearing he may look ordinary. She is dismayed when this is the case.
Algernon is passing himself off as Ernest. Cecily startles him in her manner of speaking. He claims he is not wicked, though he is reckless. Cecily is glad he isn’t a hypocrite. Algernon/Ernest regrets missing Jack. He wants to miss an appointment in London. Cecily tells him he should wait for her uncle, who wants to talk about his departure for Australia. Algernon/Ernest is not pleased. He decides he’ll have to reform.
Algernon compliments Cecily on her beauty. She claims Miss Prism says that good looks are a snare, and Algernon agrees. Cecily doesn’t desire a sensible man, for she’d not be able to converse with him.
Miss Prism returns, telling Chasuble that he should marry so he doesn’t tempt people. A married man is not attractive to anyone except his wife.
Jack enters to everyone’s surprise. He is dressed in mourning. He claims his brother Ernest died. Miss Prism believes Ernest reaped what he sowed. Jack claims he got a telegram from Paris stating that Ernest died from a chill. Chasuble offers to do a sermon in Ernest’s honor.
Jack asks whether Chasuble performs christenings. Miss Prism claims it is a luxury the poor insist on spending upon. Jack claims he wishes to be christened. Chasuble agrees and sets an appointment.
Cecily enters and tells him that Ernest has arrived. She leads Algernon in, who apologizes for his past deeds and plans to reform. Jack is angry. Cecily tries to make peace by saying that Ernest can’t be all bad since he visits invalids like Mr. Bunbury. Jack finally shakes Algernon’s hand at Cecily’s persistence.
Miss Prism calls to Cecily to come with her. Jack orders Merriman to order a dog cart, intending to end Algernon’s visit. Algernon refuses to leave. He finally agrees to but wishes to see Cecily first. He tells her she is perfect. Cecily writes down his compliments in her diary, which she hopes to publish. Algernon sends away the dog cart.
Cecily tells Algernon she fell in love with him because Jack and the others were always discussing him. She imagined a whole romance and engagement. She broke off the engagement because it can’t be serious if it isn’t broken off once. She forgave him a week later. She loves his name (Ernest), and she doesn’t like the name of Algernon. Algernon goes to Chasuble to see about a christening.
Merriman announces the arrival of Miss Fairfax, who is there to see Jack. Cecily introduces herself to Gwendolen, who likes her immediately. Cecily assumes Gwendolen works with Jack in philanthropic work. She doesn’t approve of such women, considering them forward.
Cecly tells Gwendolen she is Mr. Worthing’s ward. Gwendolen is surprised to learn this. She wishes Cecily weren’t so young and pretty. When Cecily hears Gwendolen mention Ernest, she claims she is the ward of Ernest’s brother Jack. Gwendolen is shocked to hear that he has a brother. Cecily reveals that she is engaged to Ernest, and Gwendolen claims that Ernest is engaged to her. Each woman believes their claim is the right one, and the other is a temptress. They start insulting each other. Gwendolen snidely comments on what isn’t fashionable with tea. Cecily makes it a point of giving the unfashionable things to Gwendolen. Gwendolen’s liking of Cecily has reversed. She now considers her deceitful.
Jack enters. Gwendolen asks if he is engaged to Cecily. Cecily tells her that this is Uncle Jack, not Ernest. She identifies Algernon, who also enters, as Ernest. Cecily asks Algernon if he is engaged to Gwendolen, and he says no. Gwendolen identifies Cecily’s Ernest as Algernon. Both women come together for protection when they realize they have been lied to. The men give their true names. Gwendolen and Cecily make up. Jack admits there is and never was an Ernest. Gwendolen and Cecily leave the men and go into the house.
Algernon and Jack reflect on how this Bunburying has backfired on them. Jack doesn’t approve of Algernon wooing Cecily. Algernon points out that Jack is as guilty of deceiving Gwendolen—his cousin—as he is of deceiving Jack’s ward Cecily.
Algernon starts eating muffins. He always eats when he is unhappy.
Both men find out they are going to have Chasuble christen them Ernest. Jack thinks it is ridiculous that they both be called Ernest. He claims he deserves to be christened because he never was. Algernon already was christened. Jack wants Algernon to leave, but he refuses until he is fed.