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. However, the initial enthusiasm arising from such an important arrest soon gives way to worry and concern. Carla, chief inspector of the brigade, is convinced that the terrorist leader's presence in Melilla is "no coincidence" and that it is extremely urgent to find out what he was planning to do on Spanish soil. The capture of Al Gahreeb also happens at a time when Carla is going through a bad patch in her private life: she is about to separate from Marcos, the father of her young daughter and also her subordinate at the Antiterrorist Unit.

Lack of necessary manpower and resources to face the threats causes internal confrontations inside The Unit, whose members must also deal with the personal problems caused by their exhaustive dedication to police work.

The unexpected disappearance of an infiltrated spy sets off the alarms at the Unit. The team investigates a businessman who met with Salah Al Garheeb previous his imprisonment to check on his connections and future steps of the terrorist cell that threatens Spain.

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. But contrary to what the police believe, somebody out there is helping them to organize their escape.

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 latest events in Madrid destroy the morale of the Unit, which must quickly regain control of the situation. On the one hand, they must stop Kader and Tarek, and on the other, prevent the distribution of chemical weapons through Spain to the rest of Europe.