Act 3 takes place inside the Hardcastle manor. Mr. Hardcastle enters and remarks to himself that Marlow is far from the well-mannered young man his father makes him out to be. Kate enters. The two talk of Marlow, and Mr. Hardcastle expresses his displeasure at the young man’s impudence. Kate disagrees with him, however, and claims that Marlow possesses qualities befitting a husband. The two agree that one of them must be mistaken, with Kate vowing to prove Mr. Hardcastle wrong. They exit.
Tony enters, carrying with him an assortment of jewelry: it is Constance’s inheritance. He is followed by Hastings, who informs him that he and Constance will run off in a short while. Tony promptly hands him the jewels, telling him he had stolen it from his mother’s drawers. Hastings is surprised at this and informs Tony that Constance is trying to procure the very same jewels as of the moment. Tony assures him that he’ll take care of Mrs. Hardcastle when she finds out the jewels are missing. Hastings then exits.
Mrs. Hardcastle and Constance enter. Constance tries to persuade Mrs. Hardcastle to let her wear some of her jewels. Mrs. Hardcastle, however, refuses, claiming that young women should not look so flamboyant. Tony, seeing an opportunity, whispers to Mrs. Hardcastle that she should lie and declare the jewels missing so as to suffer Constance’s pleas no more. Mrs. Hardcastle takes up Tony’s suggestion and informs Constance that she has lost the jewels. Mrs. Hardcastle then exits. Constance is not dismayed for long, as Tony informs her that the jewels are, in fact, already in the possession of Hastings. Constance exits.
Mrs. Hardcastle enters once more, in hysterics, having just found out that the jewels are missing. She informs Tony, who reacts as if Mrs. Hardcastle is merely faking. Mrs. Hardcastle tries to clarify that she is not faking this time—the jewels really have been stolen. Tony, however, continues to regard Mrs. Hardcastle with disbelief. Losing her temper with him, Mrs. Hardcastle runs Tony off the stage. The two exit.
Kate enters in her newly donned modest attire, followed by a maid. She asks the maid if she can pass as a common bar-maid. The maid answers affirmatively and then asks her why she would want to do such a thing. Kate replies that she means to deceive Marlow in order to catch him off his guard. The maid exits.
Marlow enters. Kate places herself before him, asking if he called for a maid. Marlow, successfully deceived, thinks Kate is a common bar-maid and readily makes conversation. He then compliments her looks and even attempts to kiss her. Kate points out that he is being so forward with her when, moments ago, he was so reserved around “Miss Hardcastle.” Marlow explains that, underneath it all, he is actually quite merry and popular with women, such as those found in the “Ladies’ Club” in town. The two flirt some more, leading Marlow to seize Kate’s hand. He immediately drops her hand and exits, however, when he sees Mr. Hardcastle approaching.
Mr. Hardcastle enters, enraged at what he has seen. Kate, however, asserts that Marlow has “only the faults that will pass off with time, and the virtues that will improve with age.” She asks that he give her until nightfall to prove that Marlow is, despite his impudence, a man of honor. Mr. Hardcastle gives her but an hour. The two exit.