Kvetoslav Svach, known as K, is an unusual character in the police department whose passion for medieval history and art are considered rare but harmless. After the death of someone for whom he was responsible, he is forced to resign from the police force. However, the assassination of a series of people in several of Prague's great medieval churches compels the chief of police to request that K return to help with the case. Soon K discovers that the murders are united by motives that began centuries before.