In the early 2020s, Fayçal Ziraoui, a young graduate of the École Polytechnique, stumbled upon the Zodiac case. In the late 1960s, this serial killer, who was never identified, terrorized California. He staged his murders and sent enigmatic letters containing cryptograms to the press and the police. Five decades later, Fayçal takes on the challenge of deciphering the last two codes sent by the killer, which allows him to identify a suspect.