When Floyd Macklin, an American black marketeer, escapes from a Black Maria, he vows to kill prosecutor Hans Eckert and Major Caldwell, the men responsible for his 20-year sentence. Jake sets up a meeting with Macklin at the Prater Amusement Park, but Caldwell and Lt. Hoffman arrive with the Viennese police before Jake can capture his man. Macklin rushes away and Jake is jailed as a suspect in engineering Macklin's original escape.

Jake uses a painting to find a royal emerald stolen by a convict, the painter of the portrait, in 1886. The painting leads him to a tunnel under the Hofburg Palace filled with plaster models of monuments. Jake must contend with several other crooks who are also after the emerald.