The play opens with the song "The ballad of Mac the Knife", describing the bad deeds done by Macheth, who is also known as Mac the Knife and is the main character of this opera. He always wears white kid gloves although he has committed various crimes such as murdering, stealing cask boxes, setting a fire and raping a child-bride. .
Another important character of this play, Mr Peachum, then appears. He is in his own outfitting shop for begging. Anyone who wants to beg in London has to go to his shop and pays Mr Peachum some money first. Then, he will be allowed to beg in a certain location and also be provided with the props for begging. Mr Peachum is angry to know that his daughter Polly is recently seeing a man because he does not want his daughter to marry anyone. Through his wife's description of the man, he finds out the man is indeed Mac the Knife, who is the most powerful criminal in London.
At the same time, Polly and Mac are getting married and they break into a stable where the wedding is to be held. Mac's men soon arrive and all the presents they present are stolen goods. Soon after, the Chief Sheriff, Tiger Brown, also arrives. Instead of arresting the gang, he gives congratulation to Mac. Mac explains that they are old friends for a long time and he usually gives Brown a share of his stolen goods. Before Mac's men leave, they surprise the couple by showing them the prepared bed.
When Polly returns home, she finds that her parents are extremely angry about her marriage. She argues with her parents, which ends up that her parents decide to destroy Mac. Mr Peachum is going to report Mac to the sheriff although Polly assures that Mac will not get any trouble because of the friendship between Brown and Mac, while Mrs Peachum is going to bribe the whores whom Mac visits every week to turn him in. Then Polly immediately tells Mac her parents" plan and Mac believes her warning until he sees the charges against him.
