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. ...