After coming out from jail, Roberto and my father are still friends. A friendship that persists despite betrayal. The spy within is the story of my father and the man who spied him. The story of Juan, the mediator that struggled for peace between ETA and the Spanish Government, and Roberto, the agent of the Secret Services who was infiltrated in our lives for years. But it is also the chronicle of our relationship as filmmakers with the spy, and the difficulty to catch his slippery identity.