In Marseille, an undercover detective is following Alain Charnier (Fernando Rey), a wealthy French criminal who runs the largest heroin-smuggling syndicate in the world. The policeman is assassinated by Charnier's hitman, Pierre Nicoli (Marcel Bozzuffi). Charnier plans to smuggle $32 million worth of heroin into the United States by hiding it in the car of his unsuspecting friend, French television personality Henri Devereaux (Frédéric de Pasquale).
In New York City, detectives Jimmy "Popeye" Doyle (Gene Hackman) and Buddy "Cloudy" Russo (Roy Scheider) are conducting an undercover stakeout in Bedford-Stuyvesant, Brooklyn. After seeing a drug transaction take place in a bar, Cloudy goes in to make an arrest, but the suspect makes a break for it, cutting Cloudy on the arm with a knife. After catching up with their suspect and severely beating him, the detectives interrogate the man, who reveals his drug connection.