Sarah Millwood is a prostitute who works the streets of London. She wants to seduce young, inexperienced men and con them out of money, and when she sees George Barnwell, she identifies him as a likely target for her scheme. She asks him to her home for supper, and as they are talking, she learns that he works for a wealthy merchant, Thorowgood. His success is well-known and Sarah decides to seduce George so that he can provide her with access to Thorowgood. Her seduction works; she flatters George and he is soon captivated by her, and finds himself wanting to do anything to impress her, and keep her. He agrees to steal money from Thorowgood, but when he gets home the next day, George is already having some regrets. One of these is staying out later than the curfew Thorowgood has given him, and he is certain that sleeping with Sarah is also a huge betrayal of his boss's trust.
Sarah visits George at work, but she can already sense that he has cooled. He is so riddled by guilt that he has decided to have nothing more to do with her, and being skilled at reading other people's emotions, Sarah senses that her scheme is almost dead in the water. She hastily comes up with a plan B. She tells George that because they slept together at her landlord's house, she is being evicted. George feels terrible that their sexual encounter is rendering her homeless and so sees no way out other than to steal from Thorowgood in order to pay for accommodation for Sarah somewhere else.
George feels so guilty after stealing the money that he cannot stay at his employer's home anymore. He runs away, leaving a note for his best friend, Trueman, explaining what has happened. He takes refuge with Sarah, who had not planned on having anything more to do with him since he no longer has access to his employer's wealth, but she remembers hearing him talk of a rich uncle and decides that she might have some use for him after all. She declares her undying love, and realizing that George has fallen for her deeply, plants the seeds of another scheme. This time she wants him to rob his uncle, but George protests, because he knows his uncle will recognize him. The only way around this, according to Sarah, is to murder his uncle so that he cannot be identified after the theft. George is now so blinded by his passion that he does as she demands. He goes to his uncle's home, and finding him alone, covers his face and stabs him. As Uncle Barnwell dies, he prays for his nephew George and his murderer, unaware that they are the same person. George is distraught and reveals his face to his uncle. Uncle Barnwell forgives him.
George's actions are revealed to Thorowgood, and Lucy, his servant, is tasked with watching Sarah's house until George returns. When he does, he is covered with his uncle's blood and in a terrible emotional state. Sarah is unsympathetic, and angered by his failure to take money or property. She calls the police and tells them that he has murdered his uncle, a crime for which he is quickly arrested. Lucy is not going to allow Sarah to get away with what she has done; having been aware of the grand plan since the start, she has the police arrest Sarah as well.
George and Sarah are sentenced to death. Sarah has no remorse, and blames society for her actions, because society has allowed some to be rich, like Thorowgood, and some to be poor, like her. She goes to her death as angry as she was for most of her life. George, by contrast, is truly contrite. Thorowgood visits him and provides him with a pastor who listens to his confession and helps him repent. He makes his peace with those he has wronged, and with God.
