Scene 1: Sebastian's chambers in Sir Alexander's house
Mary Fitz-Allard and Sebastian are in love, but their fathers will never permit the union, as Sebastian's father (Sir Alexander) demands too large a dowry for the girl. However, Sebastian has a plan to enable the match: he will pretend to be in love with Moll Cutpurse, a notorious cross-dressing thief, and his father will be so worried that he will see marriage to Mary as the preferable alternative.
Scene 2: The parlour of Sir Alexander's house
Sebastian's father is worried about his son's pursuing a grotesque "man-woman." Sebastian pretends to be outraged, and asks if his father would be happy if he married Mary instead. His father says no. Sir Alexander calls the spy and parasite, Trapdoor, and sets him to follow Moll, get into her services, and find a way to destroy her. Trapdoor agrees.
Scene 1: The three shops open in a rank
A scene in the street. Various gallants are talking to the shopkeepers and flirting with their wives. The gallant Laxton flirts with Mistress Gallipot in the tobacco-shop. He does not actually like her much, but is keeping her on a string in order to get money from her to take other girls out. Jack Dapper, a young profligate, enters to buy a feather to show off with. Moll enters and Laxton takes a fancy to her, assuming that because she dresses as a man, she is morally loose. Laxton courts Moll; she agrees to meet him at Gray’s Inn Fields at 3 o’clock. Trapdoor then presents himself to Moll and offers to be her servant. She is dubious but agrees to meet him in Gray’s Inn Fields a bit after 3 p.m.
Scene 2: A street
Sebastian's father is spying on him at home. Moll enters and Sebastian woos her. She is polite but rebuffs him: she is chaste and will never marry. He says he will try again later. She exits and Sir Alexander rebukes his son, saying that Moll will disgrace him, that she is a whore and a thief. Sebastian says that there is no proof of that. Sir Alexander exits, still more resolved to publicly shame Moll. Sebastian decides that he must confess his plan to her, and get her to help him and Mary.
Scene 1: Gray's Inn Fields
Laxton enters for his rendezvous with Moll. She appears dressed as a man; he goes towards her but she challenges him to a fight: he has impugned her honor, assuming that all women are whores. He is wrong to assume so—and also wrong to treat whoring lightly, as it is a sin that many good women are forced to, for lack of an alternative. They fight and she wounds him; he retreats and exits, shocked. Trapdoor enters. Moll teases and taunts him, but she finally accepts that his motives are good and agrees to take him on as her servant.
Scene 2: Gallipot's house
Mistress Gallipot is continuing her covert affair with Laxton, which her husband does not suspect. He has sent her a letter asking for a loan of thirty pounds, which she does not have. She formulates a plan, tearing the letter and wailing. She tells Gallipot that Laxton is an old suitor, to whom she was betrothed before they married. She thought him dead, but now he has returned to claim his wife. Gallipot is horrified, as he loves his wife and has had children by her. She suggests buying Laxton off with thirty pounds and he agrees. Laxton enters. Mrs Gallipot quickly apprises him of the situation and Gallipot offers him the money. Feigning anger at the loss of his "betrothed," he takes it and exits, musing on the deceitfulness of women.
Scene 3: Holborn Street
Trapdoor tells Sir Alexander what he has learned: Sebastian and Moll plan to meet at 3 o’clock in his (Sir Alexander’s) chamber to have sex. They decide to trap her. Meanwhile, Sir Davy Dapper talks to Sir Alexander about his son, Jack, who is still wild and profligate, whoring, drinking, and gambling. Sir Davy has decided to teach his son a lesson: he will arrange to have Jack arrested, trusting a few days in the counter (the debtor's prison) to bring him to his senses. The sergeants enter and Sir Davy (pretending not to be Jack’s father) gives them their instructions. Jack and his servant Gull enter, and are about to be set on by the sergeants; but Moll, who has been watching, warns him away. Jack escapes, and Moll mocks the police, who can do nothing.
Scene 1: Sir Alexander's chamber
Sir Alexander and Trapdoor await Moll and Sebastian. Sir Alexander lays out diamonds and gold in the hope that Moll will try to steal them. Sebastian and Moll enter with Mary Fitz-Allard (who is disguised as a page). Moll, in on the plan, aims to help them. She plays on the viol and sings. She then sees the jewels but only comments on them, rather than stealing them.
Scene 2: Openwork's house
The citizens' wives discuss the gallants, agreeing that they don’t really understand life, women, or relationships. A young man enters pretending to have legal document calling the Gallipots to court for breach of contract. This is Laxton’s doing — after the thirty pounds, he demanded another fifteen, and now he is asking for a hundred in order to hold his peace. Mistress Gallipot is shocked. Her husband wants to pay Laxton off again, but she feels forced by the enormity of the sum to confess that all this was a lie, that she was never betrothed to Laxton. Gallipot asks for an explanation. Laxton "confesses" that he had courted Gallipot’s wife but had been refused, but that she promised she would always help Laxton out if he were in need; thus they concocted this story to get money. Gallipot believes them; Laxton is not punished, but ends up being their guest for dinner.
Scene 1: A street
Moll tells Jack how she saved him from the sergeants and he thanks her. He knows that his father set the law on him, and is amused at the idea that time in jail would cure him, when in fact jail just teaches people how to be worse. Moll has grown to suspect Trapdoor’s honesty and has discharged him. Trapdoor enters, disguised as a soldier, but Moll sees through him. A gang of cutpurses enter to try to rob them, but Moll sees them off: she is known and feared by all rogues. Moll explains that she thinks it her duty to protect the honest through her knowledge of London’s low-lifes.
Scene 2: Sir Alexander's house
Sir Alexander hears that his son and Moll have fled to get married. Sir Guy, Mary’s father, gently mocks Sir Alexander, saying it is his fault for opposing the match with Mary. Sir Guy then offers to wager all his lands, against half of Sir Alexander’s, that Sebastian and Moll will not wed. Sir Alexander takes the bet, declaring that anyone in the world but Moll would make a welcome daughter-in-law. A servant announces Sebastian and his bride; he enters, hand in hand with Moll. Sir Alexander is horrified, but demands Sir Guy’s lands. The trick is then revealed: Mary is brought in as Sebastian’s bride. Sir Alexander, vastly relieved, apologizes to both Mary and her father, and happily grants half his lands to the happy couple. Moll, asked when she will marry, says she never will. Trapdoor enters to confess that he was set on her by Sir Alexander, who promises never again to value someone according to public opinion.