In 1971 the Knapp Commission, entrusted to expose corruption in the New York Police Department, faces a major problem. They have little physical evidence - their main source, Frank Serpico, won't wear a wire. They finally strike gold when a corrupt cop named William Phillips agrees to become their undercover agent, in exchange for immunity from a bribery charge. The evidence he eventually collects leads directly to the indictment of thirty New York cops. But the tables quickly turn when Phillips himself is convicted of a double-homicide. Three decades later and still in prison Phillips claims he's the victim of a massive police conspiracy.