Criminal profilers Jan Grall and Rabea Wyler are assigned the gruesome case of a man found murdered in a wildlife park. Later, two more bodies are found, both brutally mutilated, and authorities soon realise that the victims are linked by the letters of the alphabet tattooed on their skin.

Then the killer strikes again and Grall and Wyler receive a mysterious package containing a blood-encrusted scrap of skin marked with a letter. And when Grall's hotel room is marked with a Z, the race is on to find out who this vicious killer is. Before it's too late...