The murder of two Civil Guard officers in Castejón, Navarre, in 2004, is the starting point for one of the most emblematic police investigations in Spanish crime history. The bullets recovered at the crime scene point to a robber who has been looting banks with impunity for ten years. He is a mysterious figure who hides behind a disguise; they call him "The Loner." His capture will become an obsession for the Civil Guard and the National Police. A police thriller with numerous plot twists in which the tension never lets up.