After the death of his son's babysitter, caused by UBH, a synthetic drug that is starting to flood the market, Commissioner Max Vernet convinces his superiors to set up an infiltration, the only way to trace a production and distribution network that is as mysterious as it is dangerous. To do this, he meets Aurélie, a single mother and chemist in the police, and asks her to offer her services as a chemist to the network run from Marseille by the charismatic Jesus on the pretext of being able to improve the product. But Aurélie refuses, her family balance and her son, whom she has been raising alone since the death of her partner fifteen years earlier, matter more than anything.

Aurélie has discovered that her son Jérôme owes 50,000 euros to a dealer and has no way of settling this debt. She decides to embezzle a stock of UBH that she was supposed to destroy, but is unmasked by the IGPN. Max then makes a contract with her: if Aurélie accepts the infiltration mission, justice is ready to forgive the crime and Max will take care of settling Jérémie's problem with his dealer. Aurélie has no choice but to accept and soon finds herself in contact with Richard Le Moal, a UBH dealer who was identified by Max and his team.

When Le Moal discovers that Aurélie is in contact with the police, he tries to escape but is knocked down by a car. As he lies dying in his hospital bed, Le Moal agrees to give the phone number of the head of the UBH network. Aurélie contacts Jesus and Nikita as they have just started a turf war in Marseille with a gang of Albanians. Aurélie offers not only to replace Le Moal but also to improve the UBH thanks to her talents as a chemist…

After long weeks of waiting, Jesus and Nikita decide to call Aurélie back and arrange to meet her in Marseille. Since Jesus is wary of Aurélie, Nikita decides to test her. After checking her chemistry skills, he orders her to poison Judge Picard who has been on their trail since Jesus assassinated the Albanian leader. Meanwhile, the judge in charge of the undercover operation tells Max that Aurélie will ultimately have to go to prison even if the mission is a success.

After Max stages a fake assassination, Aurélie manages to make Nikita believe that she has indeed poisoned Judge Picard and is accepted into Jesus' hideout. Now she has to find out where the laboratory that produces UBH is before Max can stop the network. But before this arrest, Max warns Aurélie that the judge has gone back on their deal. When, during the night, Aurélie is caught approaching Jesus' computer, she kisses him...

Aurélie is now playing a double game: while she is supposed to discover the address of Jesus' lab, she chooses to confess to him that she is an undercover cop. Ready to do anything to avoid going to prison and to be able to take care of her son, Aurélie decides to run away with Jesus who promises her that they will take Jérémie with them. For his part, Max is abandoned by his superiors while at the same time the Albanians find Jesus' trail. Now, he and Aurélie are in mortal danger, while Max is officially relieved of the case...