Floyd Macklin escapes from custody after being sentenced to 20 years in prison for black marketing activities. He vows to kill the prosecutor and Maj. Caldwell, who were responsible for his capture. Jake sets up a meeting with Macklin to talk to him, but the police arrive and Macklin bolts. Jake finds himself under arrest on suspicion of helping Macklin escape in the first place.
Jake is about to deliver to Maj. Caldwell some microfilm that identifies 12 major gangland leaders at a secret summit meeting. Before he can, though, they're attacked by a gang of toughs and Caldwell is wounded.
Aline Masterson, a New York museum official, hires Jake to recover the Royal Crown of Bosnia, which was stolen from her outside the Hofberg Museum. Jake manages to find the thief who has it and arranges to deliver a payoff to him. However, when he gets to the pre-arranged spot he is attacked and knocked unconscious. When he awakes he finds that both he and Masterson are suspected of being part of the scheme themselves.
A former girlfriend of Jake's, Annalisa, contacts him and asks him to help her husband, who is in jail accused of murdering his business partner over a valuable oil lease. She claims he is innocent of the crime, and Jake's interest is further piqued when a wealthy but shady Turk warns him to stay out of the situation.
An old woman who sells flowers is killed outside of Jake's Bar, but Maj. Caldwell believes that Jake was the actual target. Jake, however, doesn't think so, and focuses his attention on one of the mourners at the woman's funeral.
Jake's search for a stolen emerald leads him to a tunnel underneath the Hofburg Palace that is filled with plaster models of various monuments. However, he finds that he's not the only person who's down there looking for the emerald.
In Vienna Jake unexpectedly bumps into Charley Stokes, a man he saw killed in Albania after they had blown up a power station. Charley tells him that he was seriously wounded but saved by Olga Compani, who was the second in command of the Albanian secret police, and that the two are now married. Jake, however, finds the whole incident fishy and suspects that the pair have something planned, and he wants to find out what it is.
Jake finds out that Frank Cort, a professional mercenary, has arrived in Vienna with a beautiful blonde. He suspects that Cort is in town to see Vasamumbo, the former leader of the African nation of Upper Namba for whom Cort commanded a mercenary army and who was deposed in a revolution, and Gen. Gundarson, the commander of UN troops in the country which is the only force strong enough to stop Cort from re-installing Vasamumbo to power.