The prologue is the "Ballad of Mac the Knife", a song that is harsh and grating to listen to in the German. It is sung while beggars, prostitues and thieves are all enjoying a fair in Soho. The ballad describes many of the things that Macheath, known as Mac the Knife, has done. He is compared to a shark with sharp teeth, but unlike a shark he keeps his weapons hidden. Mac the Knife always wears fancy "white kid gloves" in spite of the atrocious crimes he has committed. The song indicates that Macheath is to blame for killing many men, stealing cash boxes, murdering a prostitute, setting a fire in Soho that killed seven children, and raping a young bride. At the end of the song the whores laugh and a man steps out of their group. As he walks away, Low-Dive Jenny cries out that that was Mac the Knife.
The introduction of Mac the Knife immediately sets him up in contradictory terms. He is represented as a shark with bloody fins and hidden teeth, but at the same time he is described in terms of "white kid gloves". These white gloves, signs of pure hands, serve as a symbol of bourgeois society. Brecht is basically saying that Macheath covers his crimes by pretending to be bourgeois. Alternatively, this can also be interpreted as implying that bourgeois society commits the crimes and then pretends that nothing ever happened. Note that Macheath does not deny his crimes; instead, he acts as if nothing is wrong.