On September 5, 2012, in a parking lot on a forest road in the Alps, a few kilometers from the village of Chevaline, an English family and a French cyclist were found dead, brutally shot. A little girl survived the tragedy and was rushed to the hospital: the killer had left her for dead after striking her on the head. The gendarmerie secured the crime scene and began a meticulous investigation to find any traces the killer might have left behind. A phone call from the owner of the campsite where the English family was staying would change everything.

The English family killed in the parking lot has been identified: they are the Al-Hillis. Well-established in an upscale suburb of London, nothing seemed to predestine them to such a fate. But investigators quickly uncovered a family dispute that could explain the tragedy: Saad, the victim, and his brother Zaid had clashed over their late father's substantial inheritance, following his death a few months earlier. For the French police, Zaid became the prime suspect, but the English police rejected this theory. Only high-level state intervention seemed capable of moving the investigation forward.

While everything suggests that the answer lies in the United Kingdom and that the brother may have orchestrated the massacre, new elements emerge, complicating the investigation. Saad had traveled to Baghdad to reclaim a family home and made enemies in Iraq. Hidden hard drives in the family caravan contain a significant amount of confidential data about Saad’s satellite company. Additionally, the discovery of Saad’s wife's astonishing secret life adds another layer of complexity. For the first time in his career, the public prosecutor must collaborate with intelligence services to advance a criminal investigation.

Long considered a collateral victim of the Chevaline massacre, Sylvain Mollier, the French cyclist also found dead on September 5, has become the primary focus of investigators seeking to explain the tragedy. Those who might have had a motive against him are being investigated one by one. A small village, Ugine, has become the center of an investigation involving more than a hundred investigators. Sylvain Mollier was caught in the middle of a love triangle—passions that sometimes lead to the worst…

After years of investigation, the gendarmes are still facing a mystery. Everything must be started from scratch. A new prosecutor is trying to find the murder weapon, which is quite unique. It is an old Swiss weapon dating back to World War I. Even though it was produced in limited numbers, the search for this weapon is a painstaking task that stretches over months and across multiple countries. Years after the massacre, customs authorities alert the investigation unit: Swiss weapons have been identified, and their owners live near Chevaline. The GIGN is called in for a high-risk arrest—never before have they been so close to their goal.

The alibis of all identified suspects hold up, but one last unknown remains: the identity of the motorcyclist whom all witnesses claim to have seen near the scene of the massacre on the day it happened. The investigators' latest theory revolves around this motorcyclist. He might be the killer. And if he isn't, he must have either seen the perpetrator or be protecting him. A composite sketch is circulating. The man is being tracked by police forces across France and England. After searching every motorcycle helmet store (witnesses described a very distinctive helmet that was only produced in limited quantities) and scrutinizing all phone records of devices that connected to the area on the day of the massacre, the gendarmerie finally identifies the motorcyclist. His interrogation in police custody leads to astonishing twists.