Danny Ciello is a narcotics detective who works in the Special Investigations Unit of the New York City Police Department. He and his partners are called “Princes of the City” because they are largely unsupervised and are given wide latitude to make cases against defendants. They are involved in numerous illegal practices, such as skimming money from criminals and supplying informants with drugs.
Danny has a drug addict for a brother and a cousin in organized crime. After an incident in which Danny beats up a junkie to supply another junkie with heroin, his conscience begins to bother him. He is approached by internal affairs and federal prosecutors to participate in an investigation of police corruption. In exchange for potentially avoiding prosecution as well as federal protection for himself, his wife, and his children, Ciello wears a wire and works undercover to expose the inner workings of illegal police activity and corruption. He agrees to cooperate as long as he does not have to turn in his partners, but his past misdeeds and criminal associates come back to haunt him.
One of his partners commits suicide during interrogation, and his cousin in the Mafia, who saves his life on one occasion and warns him of a contract on his life at another point, winds up dead. While confessing three crimes he committed in the 11 years he worked for the SIU, Danny perjures himself by denying the many other offenses he and his partners have committed. Despite repeated professions of loyalty, he finally gives up all of his partners, one of whom shoots himself as a result of this betrayal. Most of the others turn against him. In the end, the chief government prosecutor decides not to prosecute Ciello, and he returns to work as an instructor at the police academy.