Sebastian's father, Sir Alex Wengrave refuses to allow him to marry his love Mary Fitzallard. Sir Alex's avaricious nature deems her dowry too small, and he threatens to disinherit his son leaving him with nothing should he choose to marry such a woman. Sebastian plans to fool his father into believing that he is in fact in love with the notorious thief Moll Cutpurse, hoping that, ultimately, his father will see Mary as the more preferable daughter in law.
Sir Wengrave hears of his son's intention to marry the harlot Moll. Outraged, he hires Trapdoor to spy on Moll, so he can cause her downfall. Wengrave eavesdrops on Sebastian and Moll. Deciding his son cannot marry this woman, he decides to frame her. Using information Trapdoor has given him, Wengrave resolves to tempt the allegedly felonious Roaring Girl into stealing diamonds. By spying on her, Wengrave plots to catch her red-handed. Sebastian himself has now warmed to Moll and her bold honourable spirit.
Trapdoor has told Wengrave that Moll and his son will meet for a sordid get-together. In knowledge of this, Wengrave spies on the two 'lovers'. Moll avoids Wengrave's trap, proving herself not to be a thief or a whore.
Trapdoor, aware that Moll suspects him, has escaped and disguised himself as a wounded soldier. It is not long, however, before Moll seeks him out and unmasks him. Moll explains that she was once close to a cutpurse, which is how she knows the tricks of the trade. This is where her troublesome reputation has come from. However, she likes to use this crooked knowledge not for her own gain, but to protect the just from vagabonds and thieves.
Wengrave has heard of his son's marriage, and is distraught to think that new daughter-in-law might be Moll. To Wengrave's relief, it is revealed that Sebastian actually eloped to marry Mary, his original fiancée. Moll's plan has worked; Sebastian now has his father's blessing over his marriage. Wengrave admits fault and vows never to judge a character based on scandal and rumour again, as he now sees both Moll and Mary as honourable women.