Jack Costello is a private detective in Miami and he finds out he is being followed by a man. He confronts him and finds out his name is Dumas. Dumas speaks with a French accent and he says he is a cartoonist and he needs Costello's image for his cartoon, but with a different name: Extralarge. Wendy is a black petty thief and when she steals an old pocket watch from Senator Pillinger and two men begin to shoot on her she seeks refuge with Costello.

Elizabeth is in a car with her boyfriend when suddenly she feels a pain in the stomach so strong she seizes a knife and stabs herself to death. Her boyfriend is charged with her murder and Dumas is present at his trial and he is struck by the boy's story. In the meantime Extralarge gets the task to find Kathy, the missing step-daughter of Laureen. Her father is dying of an unheard of disease and he desires to see her. The two girls were friends and a shade of Black Magic overhangs on them and their families.

Agent Blake finds the corpse of a child. It is one of a long chain. The murderer is called The Miami Killer by the newspapers. Extralarge assists the police in their investigation. All the murdered children suffered from some kind of ailment and the murderer knew his or her victims very well. The successive victim appears to be Elsie who is asthmatic and Extralarge suspects father Marthy and Elsie's teacher. Then Elsie is kidnapped. Can she be saved?