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.

The famous football player Long John is killed and the police believe that John was a heroin addict and that his murder is a punishment for something he did. His mother nevertheless swears that his son hated all kinds of dope and asks Costello to find out the truth whatever the result may be. Costello accepts the appointment and during his investigations notes that Cannon Ball, the best friend of long John, is scared. Also Gonzales and doctor Shuby seem to be involved. When his car blows up Costello gets angry.

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?

Dumas is alone in Costello's office when a girl offers him a thousand dollars to take her father, the famous scientist Klaus von Hermann, to his hotel. Dumas accepts the appointment but von Hermann is killed in front of the hotel. The scientist was working in Libya at a gas to use against ozone layers hole and he had instead dicovered a very poisonous gas. Before dieing he puts the secret formula in Costello's jacket. So Extralarge becames a moving target for everyone interested in a mass killing formula.

Burt meets Jack Costello after many years. Burt is a boxer and he is fleeing after a fixed match that he should have lost but instead he won. Then he vanishes. Costello finds in his office Burt's son, 'jo jo' Ritchie, who had came because his father had ordered him to go to him. Then Jo Jo cames back to Groggy where he lives and when the two of them are dining, two men arrive and kill Groggy. Jo Jo runs away and Extralarge sets on finding him and his father.