Agent Malotru (Debailly) is recalled to Paris after six years undercover in Damascus, and ends his affair with Nadia El Mansour. In Algiers, agent Cyclone is arrested, drunk, despite being avowedly teetotal. Debailly struggles to forget Nadia and to return to a more normal working life. He is assigned to train Marina Loiseau, who will work undercover in Iran. Debailly discovers that Nadia is in Paris and resumes contact with her. Meanwhile, agent Cyclone has gone missing in Algeria.

Debailly is still under routine surveillance, whilst resuming his affair with Nadia under his cover identity Paul Lefevbre. He starts to question Nadia's reason for being in Paris. Meanwhile, the DGSE hires psychologist Dr Balmes, and asks her to review his state of mind. Agent Gherbi is exfiltrated from Algiers after hearing about Cyclone, and Marina continues preparing for her mission. The episode closes with a brief scene of Debailly recounting his story while connected to a polygraph.

Marina starts work at the Institute of Earth Physics. A brief scene shows Debailly, connected to a polygraph, continuing to recount his story. The DGSE interrogates Gherbi to confirm he is not also working for Algerian intelligence. Duflot's brother-in-law works undercover in the Sahara Desert. Meanwhile, Nadia's visits to Lefevbre arouse the suspicion of Syrian agent Nadim. Debailly digs into Nadia's background, and confronts her as to why she is really in Paris.

DGSE makes contact with Algerians holding Cyclone, and Sisteron goes to Algeria. The Syrians continue to investigate Paul Lefevbre, testing him by mugging him in the street. They also seek help from the Russians. Meanwhile, DGSE puts Marina through a surprise mock interrogation. DGSE is also concerned that Cyclone's capture could have compromised the Sahara Desert operation, in which the Americans are involved. In flashforward, Debailly continues to recount his story to the Americans.

Debailly continues to investigate the leads on Cyclone. Nadim traces Debailly's daughter Prune and starts asking about him. Marina and another agent force a colleague at the Institute to refuse the secondment to Iran. Debailly realises that the Syrians and Russians are tailing him, and finally tells Duflot about Nadia and her involvement with the Syria talks. Meanwhile, the DGSE identifies who is holding Cyclone for ransom in Algeria.

Debailly continues to be tailed by the Russians. The DGSE meet an Algerian with information about Cyclone. They decide to recruit Nadia; Marie-Jeanne realises that Debailly has not told her the full truth, and confronts him. She also helps Marina compel a second colleague to refuse the Iran secondment, and teaches Marina a lesson about not getting close to her contacts. Debailly continues to be interrogated by the CIA in flashforward; it is revealed that Dr Balmes works for the Americans.

Nadia refuses to betray her country, and is angry at Lefevbre's revelations. Ill from the stress of her situation, she confesses to Nadim, and is flown back to Syria. The Russian FSB confronts DGSE and buys its silence. In Algeria, DGSE fails to plant a bug on the General holding Cyclone; Cyclone is handed over to ISIS. Marina is trained to deal with interrogation by Marie-Jeanne, tested by an Iranian agent pretending to be Reza's wife, and offered the Iran secondment. In flashforward, Debailly continues to talk to the CIA; the Americans do not fully believe his story.

ISIS threatens to execute Cyclone; Debailly is asked to work on his rescue. Nadim tries to blackmail him with Nadia's life, wanting him to betray Syrian sources; Debailly refuses to do so. The Americans pull out of the Sahara operation; DGSE suspects they may have a mole. Marina is further tested before leaving for Iran. Debailly shares details of the rescue plan for Cyclone with his colleagues.

Debailly asks DGSE to help Nadia, but is refused. He attracts the CIA by approaching the Syrians, purporting to represent the Americans, and takes time off to meet the CIA (revealed as the flashforwards shown from Episode Two). Dr Balmes suggests his motivation is not love for Nadia, but craving for the double life he experienced undercover. He agrees to return to the DGSE as a CIA double agent. Meanwhile, Marina disappears in Tbilisi en route to Iran, kidnapped by the Georgian secret service to see from where she seeks help. Cyclone is rescued in an ambush near the border with Mali.