Based on the true case of the "mask man", the film tells the story of Claus Jansen, whose son Hannes disappears from boarding school overnight in 1992. A few weeks later, the boy is found murdered. The father begins to investigate himself and makes the search for the perpetrator his personal mission. Claus is pushing the question of his own guilt: If he hadn't sent Hannes to boarding school, would his son still be alive today? Claus knows that if he doesn't end his obsessive hunt, he will lose himself and the love of his wife Heike and his younger son Sebastian ....