A high-speed train from Milan draws into the station in Rome, and an horrific discovery in one carriage rocks the city. Preliminary investigations are put in the hands of Deputy Police Commissioner Colomba Caselli. 

The police receive a message claiming responsibility for the act and announcing more murders to come, and they duly turn their attention to a small terrorist group of Islamic extremists. But investigator Dante Torre does not believe this angle. For him, this feels like a smokescreen concealing the actions of a killer who has a far more terrible motivation to continue. 

The trail leads to Berlin and Venice, where the waters of the Venetian Lagoon will turn blood red...