Kamila Housková (Gabriela Vránová), a student at the School of Applied Arts in Prague working part-time at an engravers' cooperative, arrives at the main police station with important information. A customer has come to the engraving workshop where she works asking them to make a cast of an old coin. The problem is that the coin, as the girl was to find out, was originally part of the well-known treasure of a ninth-century Byzantine merchant, which was supposed to have been completely destroyed by fire during an air raid in the Second World War. The young criminologist Exner (Jirí Vala) doesn't take the enthusiastic blonde too seriously but, since he quite fancies her, he finally offers to help her.