In Marseilles, France, the police officer looks for an entrepreneur, Alain Charnier (Fernando Rey), an undercover cop tailed Charnier as he was a smuggler of heroin. In Brooklyn, NY, a Santa who was a detective named Jimmy "Popeye" Doyle (Gene Hackman) was talking with some children a small hotdog vendor was also standing there. The hotdog vendor was the partner of Detective Salvatore Russo (Roy Scheider). Salvatore nicknamed as "Cloudy" by Doyle. Both the cops were Keeping an eye on the bar to find a person named Willie (Alan Weeks).
Later, Willie is seen giving drugs to his companion by Popeye, who started singing to give a signal to Cloudy. Cloudy suddenly came and caught the companion, but Willie managed to flee outside. Willie is chased by the cops and is caught and beaten by them. After that, Willie is interrogated to know about his connection with the drugs. Later, Charnier goes to his home to his seaside villa and meets his wife (Ann Rebbot). Then Charnier met with his gunman Nicoli for an old friend of Charnier who was a tv personality named Henri Devereaux, who was traveling to the US for a movie and will help Charnier.
Cloudy and Popeye saw a group of people having narcotics connection in a nightclub called The Chez and tailed them the Whole night. In the morning, both Popeye and Cloudy left for a candy store and noticed that the blonde had now worn a wig and was now a brunette. For many time, they kept an eye outside the candy store and found that the Greaser was Salvatore"Sal" Boca and his wife, Angie. Many unsavory type people visited the store daily from New Jersey, where a known drug financier bank-rolling a heroin shipment from Mexico and also known as a lawyer named Joel Weinstock lived. Walt Simonson (Eddie Egan) who was the boss of Popeye and a real life inspiration for Popeye reminded Popeye about a previous case where his humps backfired.
The federal Bureau of Narcotics and Dangerous Drugs also got involved them and appointed Bill Mulderig and Bill Klein. Nicoli, Devereaux, and Charnier reached to NYC and Charnier's Lincoln was brought by Devereaux. La Valle, who was an interpreter, found Lou Boca, buyer for Charnier's business. Several days later, the wiretap finally brought Popeye and Cloudy at the first break. Popeye, Cloudy, and Mulderig then saw Sal at the meeting with Nicoli and Charnier. Then Popeye discovered that Frog One was staying at the Westbury Hotel. A young chemist tested the sample of heroin brought by Charnier, and it was measured to be 89% pure. Popeye reached at the Westbury next day to see Charnier where they played a cat-and-mouse game.
Charnier managed to escape and runs in the train waving goodbye to Popeye. In Washington DC, Charnier met with Sal and insisted on consummating the deal. While going back to New York, Charnier told Nicoli about the worries he had. Popeye then returned to Brooklyn apartment building where Nicoli fired him from the roof of the building. Popeye managed to save himself and ran after Nicoli, but Nicoli managed to board a train. Popeye called the transit policemen to follow Nicoli, who killed the cop and also the conductor. The train then collides with a stopped train throwing Nicoli, and then Popeye caught Nicoli and shot him dead. Many thieves then they got arrested by the policemen. Devereaux met again with Charnier to which Charnier revealed he was now an accomplice.
Charnier then drives to Ward's Island where he goes to a factory, and then the stash of heroin was found and tested to be positive. It was hidden in the factory. After the completion of the deal, Bocas celebrates. Then there is a gunfight held which leads to Sal Bica's death. Popeye then hunted for Charnier. Popeye then wanted to get Frog One at any cost and entered a warehouse then a single gunshot was heard. Later in the epilog, it was disclosed that Weinstock and Bocas received sentences, and Henri Devereaux was imprisoned for four years. Charnier managed to escape and left for France. Also, Doyle and Russo got suspended from the duty of Narcotics.