In the 70's, during the last stages of Franco's dictatorship, Txema, a construction worker, is arrested because of his connection to some terrorists who have just committed a murder. The Secret Service decides to use him to infiltrate the ETA terrorist band. They try to offer him a deal. Financial problems force him to take their money and accept the mission. He adopts the undercover name of "Lobo" (Wolf) and becomes an active member of the band and acquires the trust of its leaders. He discovers that the group has deep internal divisions between those who want to abandon the armed fight and become just a political party, and those who want to keep the terrorist activity until they can proclaim their independence. After the band carries out their most daring and shocking crime yet, killing the prime minister, Carrero Blanco, the military heads in Madrid launch an immediate attack on ETA. Ricardo, the chief of the secret service, convinces them to wait until Lobo can complete his mission and enable them to dismantle the band. But when the Barcelona police manage to catch the leader of the band, the generals, jealous of this success, decide they can't wait any more and that they will override the secret service and catch whatever ETA members are in Madrid at that point, including Lobo, who is now left to his own devices to escape from both sides.