Vince Cipolla, a 20-year veteran on the police force, is dying of leukemia and wants to leave money for his wife. He gives confidential information on the locations of gambling and prostitution houses to two criminals who share their ill-gotten gains with Cipolla. Serpico follows the clues to Cipolla, who once instructed him, and tries to help.

Serpico is on the trail of arms broker Raoul Christie who is illegally selling M-16s to criminals. Serpico gets caught in a department turf war when he gets Christie's man Lincoln killed; Lincoln was an informant. Serpico befriends Christie's favorite prostitute, Erica Molinas.