Assignment: Vienna

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.

Before Jake can turn over to Major Caldwell microfilm identifying 12 gangland leaders at a summit conference, they're attacked by a group of thugs. After Caldwell is wounded in the scrimmage, Jake must leave him behind at the hospital and takes Susan with him to get the microfilm delivered.

Jake is hired by Aline Masterson of a New York museum to recover the Royal Crown of Bosnia, stolen from her outside the Hofburg Museum. Through underworld contacts Jake delivers the pay-off money to a hijacker. When Jake is knocked unconscious at the rendezvous site, both he and Aline are suspected of being confederates.

Annalisa, a former girlfriend, reenters Jake Webster's life by asking him to prove the innocence of her husband, jailed by the Vienna police for the murder of his associate in valuable oil leases. Jake's curiosity is aroused when Karafatma, a wealthy Turk, warns him not to investigate.

Major Caldwell believes Jake Webster was the real target when an elderly flower lady is killed outside of Jake's Bar. However, Jake suspects another motive. He decides to investigate one of the three mourners at the old woman's funeral.

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.