"El Lobo" es la historia de Mikel Lejarza (Eduardo Noriega), el agente de los servicios secretos españoles que consiguió infiltrarse en ETA entre 1973 y 1975 y provocó la caída de una cuarta parte de la militancia etarra de la época, incluyendo a los miembros más destacados de sus comandos especiales y a la cúpula dirigente del momento. La historia del Lobo es la de un hombre utilizado y destrozado por los servicios secretos de la dictadura que intentaron eliminarle en mitad de la operación y que tuvo el coraje de salvarse por sus propios medios y continuar con su misión.

Lobo is inspired by the life of Mikel Lejarza, Lobo (The wolf), the Spanish secret agent who infiltrated ETA between 1973 and 1975. Lobo contributed to the fall of one quarter of the terrorist activists of the organisation, including several Special Forces members and some top-level figures of the group. Lejarza s infiltration destabilised the terrorist organization at a time when its actions where becoming a perfect justification for the most conservative arm of the Franco regime to take full powers and stop the democratic process in Spain. The Lobo operation stopped terrorists plans to escape from the prison they where held at and a campaign of blind attacks. These demonstrations of power to a dying dictatorship intended to provoke the army to ensure the action repression action spiral.
Lobo is the story of a man used and destroyed by the Spanish dictatorship secret services who tried to get rid of him during the process. The Lobo operation was the most successful operation ever led by the Spanish police against ETA.
As a consequence, ETA sentenced Lejarza to death and covered the whole Basque country with search posters of the traitor. Lobo had to change his name and face and disappear without a trace. The memory of Lobo is so present to the ETA that its commando members always carry around a bullet bearing his name.