He tortures. He murders. Letter by letter. Jan Grall and Rabea Wyler, profilers at the LKA, take on a particularly gruesome case: a man was found in a wildlife park with a shattered skull. Shortly thereafter, the investigators come across two more bodies, both brutally mutilated. All the victims have letters tattooed under their skin: an A, a B, a C. And this is just the beginning. In the mail, Grall and Wyler receive a sinister message: a blood-crusted piece of skin with another letter. Is the victim still alive? When Grall's hotel room is marked with a Z, the race against time truly begins.