Act V begins in Hardcastle's house.
Hastings enters with a servant, who tells him that Mrs. Hardcastle and Constance left a while before, and must be far away. The servant also tells him that Charles Marlow has arrived, and Hastings, who still wants to avoid detection, heads to the garden even though he has little faith Tony will save him.
He exits as Hardcastle and Charles Marlow enter, laughing about young Marlow mistaking Hardcastle for an innkeeper, and Charles Marlow offers that his son will not need much fortune in the way of dowry, since he is already wealthy. Hardcastle shares that Kate believes the two young people like one another, but Charles Marlow waits to see for himself.
Marlow enters to apologize again for his impudence, but Hardcastle is able to laugh it off. They discuss his daughter, whom Marlow praises but says he did not share any intimacy with. Hardcastle, who saw Marlow take her hand in Act III, accuses him of lying, while Marlow continues to insist that their meeting was "without emotion." Sir Charles attempts to rectify the situation, but neither man understands why the other believes what he does, and Marlow leaves.
Kate enters almost right away, and the two elders interrogate her. When she answers that Marlow did indeed meet her more than once and spoke in effusive tones, Sir Charles is certain she lies, since he knows his son's manner to be "modest." There is an irreconcilable perspective amongst them, so Kate proposes they all meet in a half-hour, and the men can listen behind a screen while she confronts Marlow. All agree.
The scene shifts now to the back of the garden.
Hastings waits alone, sure Tony will not come, when the latter finally arrives, covered in mud. He assures Hastings he is "the best friend you have in the world" and explains what he's done. He drove the horses around in circles, through difficult areas, until he finally crashed the carriage into a horse-pond nearby. Thinking herself 40 miles from home, Mrs. Hardcastle is in a panic. Tony stresses that his means of conflict resolution has proved superior since no one has been harmed, and Hardcastle agrees.
Mrs. Hardcastle enters, terrified and lamenting being so far from home. She wonders whether the night could grow worse through a robbery, and almost right away, Tony points out a hat that can be spied over the bushes. He realizes it must be Mr. Hardcastle out on his nightly walk, and so exaggerates the appearance to convince his mother it must be a highwayman. He instructs her to hide in the thickets, which she does.
Hardcastle enters and is surprised to find Tony back so soon. As Mrs. Hardcastle prays to herself that Tony will come to no harm, Tony tries to dissuade Hardcastle from investigation by claiming he was talking to himself and so the latter did not hear any voices. Hardcastle persists in pushing through, which leads Mrs. Hardcastle to throw herself at the mercy of the “bandit” to save her son, at which point it takes a few passages of confusion for all to sort itself out and everyone to be angry with Tony again, although Hardcastle sees "morality" in the way he abuses his mother in pursuit of justice, and forces her to reap the spoiled nature she has sown in him.
Hastings and Constance enter, the former begging the latter to join him in eloping. But Constance, having been through so many trials in this night, no longer wants any part of duplicity and wants instead to apply to Hardcastle for leniency and permission to marry. Hastings insists he lacks the power to grant their wish (that lies with Mrs. Hardcastle), but she believes his sense of justice might lead him to use his influence on their behalf.
The scene shifts back to the house.
Here, Sir Charles laments his situation to Kate: either his son is a liar or is an impudent fellow. Kate suggests it might not be so bad, and the man retires so as to observe the meeting between the young people.
Once he arrives, Marlow again laments his situation, where his passion is enflamed by the grace and appearance of this girl who lacks the fortune to please his father. He insists he must quit her immediately, and she grants him this, herself sad that "all [his] serious aims are fixed on fortune." He assures her fortune was not what drew him to her but rather her qualities, which he is learning to see as "refined simplicity." Through his speech to himself, he resolves to stay with her despite his father's lack of approval. She refuses him, claiming such a union will surely result in resentment, but he claims otherwise, and gets down on one knee before her.
At such a move, Sir Charles and Hardcastle charge from behind the screen and each accuse Marlow of falsehood, though for different reasons. In the attacks they launch at him, the truth of Kate's identity is revealed and Marlow is immediately leveled, saying "Oh, the devil." Having been caught, Kate continues to mock him, asking which of his "characters" he intends to use now. Hardcastle softens and asks Kate to forgive him, at which point the lovers move off to speak privately.
Mrs. Hardcastle enters, claiming loudly that her niece has eloped with Hastings and that she will not ever release the girl's fortune. Hardcastle accuses her of being "mercenary" but she tells him to mind his own business, reminding him that if Tony refuses to marry Constance of his own volition once he is of age, then her fortune goes automatically to her.
As Hastings and Constance arrive to beg forgiveness, Hardcastle reveals that Tony is actually of age and pretends otherwise, and so the fortune is for Constance after all. They had kept Tony's true age a secret in hopes it might induce him to mature more quickly. As his first act of age, Tony takes Constance's hand and in a wonderful parody of a marriage proposal, swears her off as a mate. Mrs. Hardcastle complains this is all "but the whining end of a modern novel" and shows no sign of having learned anything.
Mr. Hardcastle gives a final speech wherein he hopes the "Mistakes of a Night" shall lead all to never mistake in his or her beloved such faulty qualities again.
Interestingly enough, Goldsmith's ending could easily be criticized as falling into the sentimentality he claims to want to eschew. Both pairs of lovers end up together, and virtue ends up as the reigning sentiment amongst everyone. There is truth to the claim, most of which lies in the conservative streak Goldsmith never aims to transcend. A happy match of men and women of breeding and character is something to aspire to in the play, and that never goes away.
But the play isn't so simple, and Goldsmith is sure to indicate his awareness of the perceived flaw, which he does by having Mrs. Hardcastle describe the end as a "whining end of a modern novel." The fact is, the emotional, happy ending is only engendered by the comic tools of flaunting vice. Marlow is the best example. While he certainly passes the test Kate poses for him, prostrating himself before a woman he considers below him in terms of fortune because he loves her, he nevertheless is a character of two minds. His baser nature is not subsumed, but rather he is kneeling before this girl as a modest woman he feels comfortable speaking to. One can well argue that the quick resolution of their issue does not detract from the absurdity of his character and contradictions, and that were this play to continue its story, we might find those contradictions causing subsequent problems.
Kate only acquires Marlow as an acceptable husband through classical comic acts of trickery. She must force him to confront his own vice and folly, his own assumptions about behavior and class, in order for him to truly feel anything that could lead to a happy marriage. In his reversal speech, where he decides he will stay with her, Marlow speaks as though her character or behavior has changed, but the truth is that the only change has been in how he perceives her social class. And ultimately, one is left to wonder whether the match would have persisted if he did not have such a tidy fortune as to not need a big dowry.
The theme of appearance and its fallacious nature remains very strong up to the end of the play. Both of the older fellows are unable to imagine a Marlow with contradictions; each needs the young man to be what he expects. Where the father will necessarily be disappointed to find his son is impudent as well as modest, Hardcastle will necessarily be disappointed to find a streak of modesty in a young man he had identified as forward.
Kate then marks herself as the heroine through her moderation. As the character who understands both the simplicity of the country and the sophistication of the town, she understands that life is about contradiction and excitement, and that happiness comes from embracing a bit of both sides. Nobody else – save perhaps her father to a lesser degree – ever exhibits such a strong understanding of life that Goldsmith seems to value.
This sense of complication is also inherent in Tony, who proves himself not a rapscallion, but rather the character with the most agency of all. It is he who facilitates the happy endings, this character who would have been identified by his audience as “low.” It is suspect to say Tony is Goldsmith's mouthpiece (though he will speak one of the epilogues), but the heroism of this low, tertiary character, and his necessity to the plot, helps to further Goldsmith's defense of “laughing comedy.” Tony, because he knows baseness better than anyone else in the play, knows how to best combat that baseness, and his practical jokes ultimately reveal to everyone their own base human level, and so are all forgiven.
Finally, the sentimental ending is not absolute because of the financial pragmatism involved. Constance does not lead Hastings back out of propriety, but because she knows they cannot survive without money, and so she is not willing to lose the jewels. There is in this a realistic concern that would have no place in a purely farcical comedy, and as such does the play continue to distinguish itself.
Again, Goldsmith is praised because he makes this complicated play seem so natural, and indeed, while the ending is sentimental in a way, it does not feel contrived from plot, but rather an honest expression of the play's themes: the baseness of humans, the falseness of appearance, the confining strictures of class, the importance of trickery, the value of moderation, etc. All of these themes play a part in bringing around an end that simultaneously creates a satisfying ending to a farcically overstuffed plot and leaves us with insight about humanity to dissect and consider.