Frederik Larsen (Lau Lauritzen Jr.), a sailor and has sailed since he was 15. Now he has lost the desire for a sailor life, especially after his best friend Bob was lost in a hurricane. When his ship docks at Copenhagen, shortly before Christmas, he signs off with all his hire on the pocket. He seeks accommodations in sailor home Bertel, located on the nice side of Nyhavn. Here he can stay and eat for very little money. He delivers his kitbag to the concierge Henrik (Mogens Hermansen) before he was lured by accordion music and the laughter that comes to him from the "naughty" side of Nyhavn. In the Cafe "The black sails", he accidently joins Inger (Lisbeth Movin) at the same table, whose profession is not in the phone book, although it is the world's oldest. Inger is in the company of an unemployed stoker Vladimir W. Olsen (Poul Reichhardt) that is full of scams, and has a good mood.
A woman is found strangled in her shop. Shortly after, another murder is committed. Perhaps the most terrifying is that the both girls' name is Sonja. The police is convinced it's the same killer and at each scene is heard a woman singing the same song. A song which is soon dubbed the 'murder melody'.
