Father Castell is dispatched to a crime scene by Papal Secretary of State Scarpia in Rome. Berthold Stollberg, a historian at the Museum for Munich History, was found dead beneath a bridge. The victim apparently knew the whereabouts of the legendary and priceless "merchant's treasure" and had hoped to meet with Scarpia to consult important documents in the Vatican. He would have liked to see the treasure return to the hands of the Holy Mother Church. To solve the crime and perhaps also find the treasure, Castell asks Marie Blank to help him.
The victim truly did devote himself heart and soul to searching for the "merchant's treasure." Castell and Blank find many notes concerning the treasure in the victim's apartment and office; yet the museum's director, Dr. Albert Kirchlechner, apparently knows nothing about Stollberg's treasure hunt. Can it be that Stollberg didn't tell a living soul about his search? While Castell is busy looking for the treasure, Marie interrogates Antonia Welz, a museum colleague of Stollberg's and, as it turns out, his secret escort on his trip to Rome. Stollberg seems to have been in love with Antonia. She led him along and accepted his attentions in order to break away from her dead-end marriage with Thomas Welz, the depot administrator of the museum. Antonia also claims to have known nothing about the search for a treasure. Father Castell and Marie see no other way of solving the crime than by going on the treasure hunt themselves. They will follow the annotations left by Stollberg in the hope that they will lead them on the track of the murderer.