In Old Bach Is Come, Empress Maria Theresa of Austria has sent secret agents Paul Müller and Franz Braun to Prussia to assassinate King Frederick the Great. The King’s preoccupation with the visiting musical master, Johann Sebastian Bach, offers a host of opportunities for the two secret agents to make inroads. However, the pair mistakenly underestimates the unscrupulous and cruel Prussian Minister of Police, Baron Manfred von Hochenheimer.
Paul and Franz concoct plans to deceive the Minister, but their trickeries merely thwart their own efforts. The two conclude that, in order to kill Frederick the Great, they will have to first eliminate the man who so staunchly protects the royal subject. They carry out a revised strategy and unexpectedly receive aid from an insurgent on his own crusade. However, each small success Franz and Paul achieve seems to set them further back from their ultimate goal of slaying the King.
Pursued by Prussian border guards, they are forced to flee Berlin on foot and seek the guidance of their prudent spymaster. In the safe haven of Vienna, the three men employ various tactics for gaining intelligence on the King. Ordered back to Prussia to finalize their mission, Paul and Franz must fool overzealous innkeepers, foresters, soldiers and secret police. Can they outwit the Prussian forces and attack the throne? Or will their fraudulent behavior have a rebound effect that propels them toward a future of torture and execution?