During a routine operation in Melilla against an Islamist network devoted to recruiting young people to their cause, the Antiterrorist Unit of the Spanish Police unexpectedly catch Salah Al Gahreeb, an outstanding spiritual leader of ISIS.

The leader of the terrorist cell, Kadir, and its two other members, cross over into Spain after stealing explosives at a mine in the Pyrenees. They hide in the town of Salt, in Catalonia, where they await orders to strike.

The unexpected disappearance of Myaz sets off the alarms at the Unit. Their spy was based in Salt, near Badalona (Catalonia), so Marcos focuses his team's efforts in that particular region while strengthening overall surveillance of the Islamic community and the work of infiltrators. However, no progress is made and Marcos feels responsible for what might have happened to their mole. At the same time, in Melilla, Miriam finds out that Salah Al Garheeb met a local entrepreneur during his stay there. The businessman, Ismail, apparently has no links to Islamic terrorism but is suspected of being a drug trafficker. It seems impossible to get close to him without arising suspicions, but Miriam finds a weak spot in Ismail's environment his own wife, Amina.

Marcos heads the party that searches for the two terrorists all over the area. Now that the fugitives no longer have any weapons or any logistic support, he thinks that they'll soon catch them.

Sala Al Gareb commits suicide inside the isolated prison cell where he has been confined. As far as Carla's bosses are concerned, his death probably means the end of a complex and uncomfortable situation. However, Carla thinks that there might be a subtle, pre-planned purpose behind his suicide. She thus discovers that Al Garheeb was able to get in touch thanks to his lawyer, with other convicts that sympathized with the Islamists' cause before he took his own life. Accordingly, all suspicious inmates are isolated. But by then, some of them have already been released from prison. Miriam finally succeeds in setting up a meeting with the informer of the CIA who had seen Al Garheeb in Nigeria. She flies to Lagos where she meets the man in question: William, an arms dealer.

The men and women from the Unit feel demoralized, deeply grieved and frustrated by their inability to prevent the terrorist attack in Madrid, but the investigation must continue.