Debailly is prisoner of IS in Iraq. DGSE asks Nadia to negotiate his release through an intermediary Shahanah, a corrupt art dealer, and promises her an EU Deputy Commissioner position in return for her help. Sisteron is sent to Iraq to meet Kurdish fighters and rescue him, but their attempt to do so during a sandstorm fails. He finds an SNCF railway points diagram left behind by IS, which DGSE believes is an attempt by 'Cochise', someone senior within IS, to communicate with them. Meanwhile Marina is working as a seismologist and is still traumatised by her experiences in Iran.

One of Debailly's jailers is a Russian FSB mole and offers to help him. Meanwhile, Duflot tries to help Marina overcome her trauma. Shahanah takes Nadia to meet Lely, a female intermediary for IS. Sisteron and Kurdish fighter Esrin are ambushed by snipers and she is injured, and flown to France for treatment. Sisteron suggests to Duflot that Esrin should be recruited in exchange for help with her son's education. Marina, plagued by panic attacks, resigns from the DGSE. The FSB mole is seriously injured in combat.

Debailly loses hope after his FSB helper is wounded, and stops eating. Marina is approached by Phillipe, a Mossad agent who is posing as DGSE and wanting to recruit her. DGSE advises her to accept the offer, and report everything back to them. Shahanah requires Nadia to launder stolen artefacts from Syria in return for his help with the negotiations, but she refuses to compromise herself. The FSB mole recovers in hospital, and returns just as Debailly is being filmed for a new hostage video by IS.

The CIA wants to bomb the camp where Debailly might be held, but DGSE insists they wait. Duflot wants to get Debailly released before he retires, but MAG reminds him that the agency doesn't normally make much effort to rescue disloyal agents. DGSE use Prune to pressure Nadia into agreeing to resume negotiations; Nadia meets Lely again and is given the hostage video. DGSE see that the points diagram has been altered to communicate that Cochise wants to defect. Duflot volunteers to go into the field and handle both cases himself. In Iraq, the FSB spy escapes into the desert with Debailly, with IS in pursuit. Meanwhile, Mossad persuades Marina to speak at a seismology conference in Baku. Simon, Marina's ex who works for DGSI, offers his help if she runs into trouble.

Shahanah is murdered by the Iranians. DGSE sets up a secret meeting between Duflot and Cochise near Jarabulus, and Duflot prepares for his mission. Lely asks to meet Nadia again, and DGSE suspects IS may have lost Debailly; their agents in Iraq confirm he has escaped. Meanwhile IS catch and kill the Russian spy, but Debailly manages to evade capture.

Debailly manages to escape into the hills, and seeks help from a farmer who betrays him to IS, and he is recaptured. French agents identify the area where he escaped, but arrive too late. In Baku Marina gives her presentation to the conference, and is offered a job there, which Mossad urges her to take; she agrees after consulting DGSE. She panics when asked to take a polygraph, but is trained by Mossad on how to pass. In Syria, Esrin takes Duflot to his clandestine meeting with Cochise; however, Cochise is followed across the river by two of his men.

Cochise refuses to spy for DGSE within IS, wanting exfiltration. Duflot agrees, if Cochise can free Debailly. Before he can return, Duflot is shot by one of Cochise's men; he is taken to hospital but later dies. Cochise obtains custody of Debailly. Meanwhile DGSE identify that Mossad plans to use Marina to infect the Iranian nuclear programme with a computer virus, which will subsequently allow them to disrupt it, via the seismic monitoring system linked to Baku. Marina panics again when she discovers the Iranian connections of her new security manager, and tries to flee but is intercepted and reassured by the Mossad agent.

DGSE mourns the loss of Duflot. Meanwhile, Mossad gives Marina the memory stick with the virus, which DGSE manages to copy before Marina successfully introduces it into the system. In Iraq, Cochise and his IS fighters take Debailly toward an arranged ambush with French special forces. Before they arrive, one of them becomes suspicious and stops the convoy; the CIA makes a drone strike on the support vehicle. Cochise is wounded in the fighting; the French forces arrive in time to rescue Debailly, who is put on a ship to Libya. The CIA insist on debriefing Debailly first; DGSE reluctantly agrees on condition that Dr Balmes does the interview. Later, Debailly escapes from the ship.

DGSE discovers that Debailly is using the stolen IDs of former agents to travel back to France. Nadia hears from her assistant's Iranian contact that Debailly is free, but DGSE refuses to tell her anything; they use Prune to tempt Debailly into making contact with her. Debailly uses burner phones to contact Prune, and then gets a message to Nadia. Meanwhile, the Mossad agent tells Marina that her mission is completed. Before he leaves he realises that Marina has not been telling him the whole truth about herself, and begins to suspect that she is a spy. He confronts her and holds her captive. Marina uses their agreed code to call for Simon's help.

DGSE tracks Debailly's attempts to contact Nadia. Dr Balmes meets her, and explains how Debailly got her released from captivity. Debailly sets up a plan to meet Nadia in a hotel, contacting her via her assistant's mobile, which DGSE is already monitoring for another operation. Meanwhile, Simon tells DGSE about Marina's call for help; he suspects she is being held in the hotel, and plants a microphone in her room. The agent is told that a Mossad team is coming to deal with her, which DGSE overhears. Marina is saved at the last minute when DGSE blow her cover. In Syria, the French pull out of helping the Kurds, but leave Esrin in place; she is killed in the fighting. In France, DGSE arrive at the hotel to apprehend Debailly, but Nadia warns him using their agreed code. Nadia is taken away, and Debailly disappears into the night.