Based on the novel by Hannelore Cayre, the story centres around Patience Portefeux (Isabelle Huppert), an underpaid, overworked French-Arabic translator in charge of phone surveillance for a narcotics police unit. When she realises she knows the mother of one of the drug dealers, she decides to cover for her and gets herself more and more involved in the world of drug trafficking. Soon, she is using her insider knowledge and police resources to build her own crime network, and earns the name Mama Weed.