It stars Eduardo Noriega as the title character and José Coronado alongside Mélanie Doutey, Silvia Abascal, Santiago Ramos, and Jorge Sanz.

The film is based on the life of Mikel Lejarza (alias 'El Lobo'), an agent of the Spanish intelligence service in the early 1970s.

During the last rales of the Francoist dictatorship, Lejarza infiltrated ETA, a paramilitary group seeking independence for the Basque Country.

He destabilized the organization when it was a potential justification for conservatives to seize power and stop the democratisation process.

ETA sentenced him to death and posted his pictures throughout the Basque Country, in the hope of someone reporting his whereabouts.