In New York City, Max Thursday, an alcoholic, down-and-out former police officer, works as a house detective in Smitty's run-down residence hotel. One day, Georgia, Max's ex-wife, arrives at the hotel and tells him that their son Jeff has been kidnapped. She also tells him that Jeff was last seen with her brother, Fred Mace, who has also disappeared. When Max asks her why she has not yet notified the police, she tells him that she has been threatened with harm by Dr. Elder, her brother's disreputable associate, if she reports the crime. Max quickly sobers up and pays a visit to Elder, who greets him at gunpoint. After drugging Max, Elder knocks him unconscious and throws him into the street. Max regains consciousness in a jail cell, where he learns that Elder has been murdered and that he is being held as a suspect in the crime. Captain Tonetti, head of the homicide bureau and an old friend of Max's, releases Max when Georgia provides an alibi for her ex-husband. Max's investigation into Elder soon reveals that the doctor was involved in a diamond smuggling operation headed by Otto Varkas. Max finds Varkas at his warehouse and threatens to tell the police that Elder was one of Varkas' men unless Varkas tells him where he can find Fred. Varkas refuses to help Max, but on his way out, Max meets Angel, a Varkas moll, who inadvertently reveals that Fred is being held captive by Varkas' men. Angel also tells Max that Elder was the pick-up man for a mysterious figure named "Saint Paul." While on his way to Varkas' hiding place, Max is shot and seriously wounded by two of Varkas' men. Despite his crippling injury, Max manages to return to Varkas' warehouse, where he finds Varkas dead. A pack of cigarettes belonging to one of Varkas' men provides the clue that leads Max back to Smitty's hotel, where he discovers that "Saint Paul" is none other than Smitty, who used the kidnapping of Jeff to force Fred to give her the jewels he got from Varkas. Smitty tries to involve Max in her scheme, but when she realizes that he has gone straight, she tells Max where to find Fred. Fred reveals the whereabouts of Jeff, and the boy is returned safely to his mother.