On the eve of Halloween, the body of a young woman is found on a railway track, embedded in the front of a train. Lieutenant Audrey Valenti is tasked with the investigation along with a brigade specializing in crimes involving the underground or railway networks. Doctor Chris Kovak, who no longer works with the brigade, has become agoraphobic and only practices medicine by teleconsultation. Then he starts to receive strange messages… The Dog, the ruthless killer with whom he has already had dealings, follows a simple principle: each time he contacts the doctor, someone dies. And contact is becoming more and more frequent…