The action in The Sentinel alternates between the present and the dark winter of 1953, where Comandante Leo Guzmán is head of a secret police unit whose job is to hunt down enemies of the state. Guzmán's brutal tactics are simple and effective: everyone is suspect.
In 2009, on a blazing summer's day in the hills near Madrid, Ana María Galindez, a Guardia Civil forensic scientist, examines what she thinks is just another Civil War grave. Fifteen bodies are hidden in an abandoned mine shaft, and forensics reveal that the killings took place sometime in the early 1950s. The identity of the suspected murderer? Comandante Leo Guzmán.
Intrigued by Guzmán's disappearance, Galindez becomes obsessed with tracking down this shadowy killer. As she investigates, she disturbs things which have lain dormant for almost 60 years. Things so compromising that those concerned will stop at nothing to destroy them - and her.