The screenplay, based on a novel by Auguste Le Breton, explores the contemporary drug scene in Paris and the efforts of the police to limit it.

After a spell in the US, master criminal Henri "le Nantais" returns to Paris and is recruited by Paul Liski, head of a major narcotics ring, to improve efficiency of distribution.

Henri continues his investigations into weak distribution links, visiting among other sordid spots an opium den and a club for black smokers of marijuana.

When Catalan and Bibi are sent to eliminate an unreliable dealer, they walk into a police ambush and escape albeit wounded, after killing two.

They immediately go to Le Troquet and demand that Henri take them to Liski, whom they want to provide them with a place to lay low.