In New York City, police officer Frank Serpico, who has been shot in the face, is rushed the hospital, where Capt. Sidney Green places a twenty-four-hour-a-day guard at his door. While the doctors work to revive him, Frank, whose friends call him "Paco," thinks back to his first days on the force in the early 1960s: Upon his graduation from the police academy, Frank and his Italian-American family are thrilled. He joins a Bronx division and immediately makes waves with his brash demands and iconoclastic manner. When a rape in progress is called in, Frank hurries to respond, despite his partner’s objection that the call is not in their jurisdiction. At the scene, three men are assaulting a woman, and one warns the officers to put down their guns or he will slash her with a knife. When they flee, Frank chases one and makes the arrest. At the station, the girl recounts her ordeal quietly, after which the boy is badly beaten during his interrogation. Frank, who declines to hit the boy, later takes him out of chains and into a coffee shop to persuade him gently to confess the names of his cohorts. Frank then tracks down the other two men at a park, but because the case is officially under the supervision of a superior officer, he cannot obtain backup. Undeterred, he manages to arrest the two by himself and brings them in for booking, but is informed that the “collar” will be credited to the superior officer, as Frank is merely a patrolman. Soon, he applies to join the Bureau of Criminal Investigation, having learned that it is a handy route to earning a detective’s shield. After two years in the department, however, Frank remains an outsider, working harder than the other officers and affecting the look of the street hippies amongst whom he works in plainclothes. Despite his close family ties, he prefers his Greenwich Village apartment to their traditional life in Little Italy, where his father and brother Pasquale run a shoe repair shop. In his new neighborhood, he buys a sheepdog puppy and takes literature classes in Spanish, the language of many of the local criminals. In class he spots Leslie, who identifies herself as a dancer-singer-actress-Buddhist, and soon they are dating. Despite their growing attraction and his inclusion in her world of upscale artists, at work Frank is increasingly alienated, and when he reads the autobiography of ballet dancer Isabella Duncan, he is accused of being a homosexual by Lt. Steiger. Furious, Frank complains to the captain, and is consequently transferred to the Bronx, where his long hair and beard set him apart immediately, but his new captain, Tolkin, concurs that his appearance allows him more cover on the streets. His fellow officers, however, consider him an enigma, and during one shootout, almost kill Frank, not recognizing him as an officer. When the uniformed policeman then asks Frank if he can claim the arrest as his own, Frank agrees with great reluctance, frustrated with the incompetence and negligence all around him. At home, when Leslie threatens to marry her ex-boyfriend, Frank allows her to go. Soon after, he becomes friends with a charismatic fellow officer named Bob Blair, who has managed to move up in record time thanks to his political acumen. Frank is continually passed over for promotions, however, and one day is transferred to the 9th district. On his first day there, he receives an envelope full of cash, and not wanting to be part of the division-wide extortion, calls Blair for advice. Together they visit Blair’s friend, Inspector Kellogg, who counsels Frank over lunch that if he values his job, he will take the money quietly. Months later, Frank is dating his neighbor, a nurse named Laurie who shares his love of classical music, and struggling to gain a transfer out of the division, where he is assumed to be “on the take,” although he is donating the extorted monies to charity. At his urging, McClain moves him to the 7th division, which the captain, Palmer, vows is ethical. Working undercover there, he meets his old friend, Tom Keough, but is disappointed to learn that Keough is instrumental in the division’s extortion ring. Frank is partnered with Don Rubello, the “bagman” who collects bribe money from bookies. Despite Keough’s warning that Frank is considered by the officers to be untrustworthy because he does not take bribes, Frank refuses the monthly payout, allowing Rubello to hold his portion secretly. Although even Laurie suggests that Frank might be better off going along with the corruption, he remains steadfast and informs McClain of the goings-on. McClain passes the information on to Commissioner Delaney, but although the commissioner claims to be delighted with Frank’s integrity, nothing comes of the interaction, and Frank worries that the other officers will turn against him. He is paired with a new bagman, Alonso, a weary old-timer who once tried to quit but was threatened into staying. When Frank admits that he does not accept bribes, Alonso informs the others, and realizing the danger he is in, Frank demands that McClain introduce him to Delaney. When McClain refuses, Frank agrees to Blair’s suggestion that they go to Jerry Berman, an aide of the mayor. Although Berman is impressed and excited, he soon informs them that the mayor has declined to investigate, afraid of alienating the police force. At home, Frank rages to Laurie that the system is crooked from the top down, and fights with Blair, whom he blames for putting Frank in more danger. Soon after, his fellow officers call him to a meeting in the park, where they are furious to learn that Rubello pocketed all of Frank’s portion of the bribe money. Though Keough urges him to accept at least a token payment to prove his collusion, Frank refuses, earning their enmity. When he complains again to Laurie, paranoid and angry, she breaks down in tears, stating that she loves him but he has grown impossible to live with. One day, Frank arrests a loan shark who has been paying off the department. When he brings the man in for arrest, the loan shark scoffs at his earnestness, considering himself well protected. Despite the attempted intervention of Keough and the others, Frank throws the loan shark in jail, then in a rage brandishes the man’s record, which states that he once killed a policeman. Meeting McClain in secret, he demands to be moved and admits he spoke to the mayor’s officer. A furious McClain demands that he keep quiet, but soon the Deputy Chief Inspector calls him in to Capt. Palmer’s office to discuss his allegations. Although they are more concerned about the possible stain on the department than on fighting the corruption, they bring his complaints to the Commissioner, who agrees only to an internal investigation. Frank, who realizes that the department cannot investigate itself objectively and that only a few low-level policemen will bear the brunt of the inquiry, wants to stay uninvolved with the proceedings, so remains vague until Capt. Green takes over the case. Frank is finally won over by Green’s insistence that Frank testify and his reputation as an honest cop who has remained scrupulous despite intense pressure. At home, meanwhile, Laurie grows exhausted by his depression and fear, and leaves him, in spite of his last-minute avowal of love. During shooting practice one day, Keough warns Frank that his life is in danger. The grand jury hearing begins, during which Frank grows frustrated that the district attorney, Tauber, is not digging hard enough to convict the top players, such as Delaney and the mayor. Although Tauber insists that Frank will earn his detective’s shield for his actions, Frank declares that it is not worth his life. Soon, he is transferred to Manhattan, but refused a promotion. In the new division, he works with Lombardo, an undercover officer who admires Frank’s honesty. When they try to arrest a bookie, however, they learn that the bookies are paying off the police, and bring their discovery of the city-wide corruption to their chief, who refuses to get involved. Incensed, Frank calls Blair and, with Lombardo, they contact The New York Times , which then launches an investigation into the corruption. Frank is transferred to the Brooklyn narcotics division, where the plainclothesmen inform him that they regularly pocket tens of thousands of dollars in drug money, and warn him that if he objects, he will be killed. Soon after, he is with three other officers, tracking a drug pusher. When they finally locate the criminal, the others send Frank in first, and when Frank tries to force his way into the man’s apartment, the officers hang back, leaving him stranded. Trapped in the door, he is shot in the face. Back in the present, while Frank recovers in the hospital, the surgeon informs his parents that, although he will suffer long-term pain and hearing loss, he is stable. When Green visits, Frank requests that the guards at his door be sent home, as they distrust him, and shows Green the hate mail he receives daily. Green hands Frank his new detective’s shield, but Frank refuses it, breaking down in tears. Later, Frank testifies before the 1971 Knapp Commission, called in response to The New York Times series, stating that he hopes that his action will save future officers from banishment and reprisals, and urges the formation of an independent investigative body dealing with police corruption. After resigning from the police force, Frank moves out of his apartment and sits on the dock with his dog, waiting for a boat to take him away from New York.