A pair of NYPD detectives in the Narcotics Bureau stumble onto a heroin smuggling ring based in Marseilles, but stopping them and capturing their leaders proves an elusive goal.
 
This is a dated movie by today's standards, but authentic to the time.  I'm not a fan of the way the film ended, but Hackman & Scheider's performances make up for it.
 
FUN FACT:  The scene where Doyle and Russo chase down the dealer near the beginning and Gene Hackman shouts out his famous question "Did you ever pick your feet in Poughkeepsie?" is based on actual "good cop/bad cop" interrogations by the real "French Connection" detectives, Eddie Egan and Sonny Grosso according to William Friedkin in the DVD commentary. Grosso would gingerly ask a suspect direct questions about his crimes, then Egan would always butt in and yell unusual questions like the Poughkeepsie one. The suspect would get so rattled by Egan's offbeat questioning that he felt more comfortable answering Grosso's, thus tending to eventually incriminate himself.