Based on a true story. In the 70's, during the last stages of Franco's dictatorship, Txema, a basque construction worker, is arrested because of his connection to some terrorists who have just committed a murder. The secret service see in him an ideal candidate to infiltrate the terrorist band ETA and become a mole, so they try to offer him a deal if he will do so. At first he's not too interested, but his financial problems (probably caused by the secret service itself) finally 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, making all the right connections until he reaches the top and acquires the trust of its leaders. In the process, 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 the independence of the Basque country. These divisions and power struggles are often resolved bloodily. After the band carries out their most daring and shocking crime yet, killing the prime minister Carrero Blanco, the military heads in Madrid become impatient and wish to launch an immediate attack on ETA and the basque country. 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 (who had been ratted out by another member who wanted his position), 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, dead or alive. And that includes Lobo, who is now left to his own devices to escape from both sides...