She Stoops to Conquer follows the mishaps of a single night in which young suitors, Marlow and Hastings, are led to the Hardcastle household under false assumptions.

At the beginning of the play, Mr. Hardcastle informs his daughter, Kate, that his friend Sir Charles Marlow has a son—also named Charles Marlow—who is to visit. He tells her, too, that he is interested in seeing Kate and Marlow marry. Kate, for the most part, is enthusiastic about the handsome Marlow but expresses doubts upon hearing of his shyness around upper-class women.

Meanwhile, Mrs. Hardcastle has been attempting to force her son, Tony, and her niece, Constance, into marriage. Kate remarks that this must be due to her mother's fixation on Constance’s fortune. In any event, her brother, Tony, is completely disinterested in Constance. Constance divulges the name of her suitor to Kate: George Hastings, an inseparable friend of Marlow’s.

At the pub, Tony overhears that Marlow and Hastings are lost. Tony schemes to repay some insults from his stepfather by intercepting the two and directing them to his father's house under the pretense that it is an inn, implying that the real Hardcastle home is still a great distance away.

Back at the Hardcastle household, Mr. Hardcastle is directing his servants how to behave. Marlow and Hastings arrive, under the impression that the Hardcastle manor is an inn. Mr. Hardcastle attempts to entertain his guests but, being perceived as a lowly innkeeper, is largely ignored and rebuffed, much to his surprise.

After Marlow asks Mr. Hardcastle if he can see his room, Hastings is left alone, and Constance overhears him referring to the house as an inn. She disabuses him of the notion, deducing that he must have been tricked by Tony. Hastings then asks Constance to elope with him at once, but she is hesitant to leave without possession of her jewels. She promises to go with him as soon as she has them. Hastings tells Constance that, in the meanwhile, they ought to tell Marlow nothing, lest he be driven by embarrassment to suddenly flee from the place.

Hastings tells Marlow that, by coincidence, their mistresses Kate and Constance have just alighted at the same inn. Alone with Kate, Marlow is consumed by nervousness to the point of stumbling upon his sentences and he scarcely looks at Kate in the face. He promptly makes an excuse then leaves. Kate remarks that if he weren't so nervous she would actually like him.

Tony, Hastings, Constance, and Mrs. Hardcastle return. Constance tries to flirt with Tony while he pulls away. Hastings remarks that the two already seem like a married couple, but Tony protests. Hastings asks permission from Mrs. Hardcastle to rectify Tony's opinion, and so Constance and Mrs. Hardcastle exit. Hastings admits to Tony that he is in love with Constance and enlists his help. Tony gladly agrees.

Meanwhile, Kate has switched to her more old-fashioned evening wear and meets her father. They compare their opinions of Marlow, both of which are unfavorable. Mr. Hardcastle finds Marlow overly impudent, whereas Kate finds him overly timid. Kate convinces her father to give Marlow a second chance to produce an explanation for these contradictory impressions.

Hastings is nearly ready to leave. Tony arrives and gives him the casket of Constance's jewels. Meanwhile, Constance is trying to convince Mrs. Hardcastle to allow her to wear her jewels. Tony interferes and suggests to Mrs. Hardcastle that she tell Constance her jewels are missing. Mrs. Hardcastle does and tries to console Constance by assuring her that she will be lent some garnets. Once she leaves, however, Tony explains to Constance that he had already stolen the jewels and given them to Hastings. When Mrs. Hardcastle returns, she is upset to find the jewels really missing. Tony taunts her and Mrs. Hardcastle chases him off.

Kate, who is talking to the maid, has discovered that Marlow has mistaken their house for an inn. Moreover, the maid divulges that Marlow has mistaken Kate for a barmaid due to her old-fashioned evening wear. Kate remarks that Marlow must not have seen her face very well because of his earlier nervousness. She decides to trade on his impression in order to test his true character. When Marlow arrives, Kate speaks to him in a different accent. He remarks on her beauty, flirts with her, and tries to kiss her, but she rebuffs him. Eventually he ends up seizing her hand. Hardcastle enters in surprise. He thinks about kicking Marlow out, but Kate pleads for another chance to demonstrate a different side to him. Hardcastle gives her an hour to do so. Meanwhile, Constance informs Hastings that Charles Marlow is to arrive later the same night. Hastings remarks that they ought to leave before then. It is then revealed that Hastings entrusted the casket of jewels to Marlow earlier, but Marlow has, in turn, cluelessly entrusted them to Mrs. Hardcastle. Hastings conceals his disappointment and decides to try to persuade Constance to abscond without the jewels.

Mr. Hardcastle argues with Marlow over the conduct of his servants, eventually exclaiming that he never expected this sort of impudence from his father’s son. Marlow, puzzled by this statement, asks a passing Kate who she is exactly. Kate pretends to be a poor relation of the family. Marlow asks if she works at this "inn" and Kate corrects him, saying that he is at the Hardcastle home. Marlow, distraught over the realization, decides to depart immediately. Kate pretends to weep, an act which touches Marlow.

Meanwhile, Tony and Constance discuss her current plans with Hastings. Mrs. Hardcastle enters and the two pretend to flirt. A letter for Tony arrives from Hastings. Constance recognizes the penmanship and decides to distract Mrs. Hardcastle. However, Tony is illiterate and so, unable to decipher the penmanship, asks his mother to read it for him. Mrs. Hardcastle uncovers their plot and declares that she will immure Constance at her Aunt Pedigree's house. Constance asks Hastings to wait for her, even though it may take years.

Sir Charles Marlow is seen with Mr. Hardcastle, laughing over the younger Marlow’s blunders. Mr. Hardcastle is happy to report that Kate and Marlow will marry, as he has seen something between the two. They ask Marlow, who contradicts this, insisting that nothing happened between him and Kate. They then ask Kate, who insists the opposite. Kate asks the two fathers to hide behind a screen to see proof of Marlow's feelings.

Outside, Tony reveals that he has driven his Mother and Constance in a circle. Mrs. Hardcastle is confused and terrified, thinking that they are lost. Hastings rushes to Constance and tries to convince her to elope, but she complains that she is too tired. She explains that she has resolved to plead with Mr. Hardcastle instead, who might perhaps convince his wife to give her the jewels.

Inside, Marlow approaches Kate, whom he still thinks is a bar-maid. Marlow is pained to be leaving her, but explains that he wants to maintain his social status. A little later, Marlow changes his mind and decides that Kate is worth more to him than his reputation. He kneels in front of her. The two fathers interrupt the scene, asking why Marlow lied earlier. Kate’s true identity is revealed, and Marlow is embarrassed once more. Mrs. Hardcastle enters and remarks that Hastings and Constance must have eloped. Hastings and Constance enter a little later, and Charles recognizes Hastings and tells Mr. Hardcastle he is a good man. They plead with Mr. Hardcastle, and so Mr. Hardcastle reveals that Tony's real age had been concealed by his mother. He is actually of legal age and can refuse Constance's hand. At the end, Constance becomes free to marry Hastings and keep her jewels, and everyone except Mrs. Hardcastle celebrates.