A man is brutally murdered in a dark alley of the oldest part of Perpignan, a sleepy town of Southern France near the Spanish border. He has the bloody mark of a triangle, carved on his cheek, a symbol that intrigues the police especially young Dominique d'Astié, recently promoted Lieutenant on the force.

A mysterious coded note and a brochure about the Holy Thorns of Christ are found in the victim's pockets. These clues lead the criminal investigation on an esoteric trail, which will involve Freemasons and other secret Brother-hoods.