During the Cold War, Michael Newman is vacationing in Venice. There, he notices a woman who's been following him for seven days. When he finally talks to her, she confesses that she is the wife of Paul Trégard, a French intelligence officer who has saved Michael Newman father's life 20 years earlier. Now, Paul Trégard is a fugitive and accused of treason by the French authorities but his wife doesn't believe these false accusations. She begs Michael Newman to help prove her husband's innocence. Since Paul Trégard saved Michael father's life, Michael feels indebted to him. She accepts the offer to investigate the whole affair. Paul Trégard is rumored to be hiding somewhere in Venice. But first, Michael talks to Paul Trégard's superiors in Paris, especially with Colonel Vallier, the chief of French intelligence. Vallier is convinced that Trégard is a traitor. Nevertheless, Michael Newman returns to Venice where he begins his investigation. He must locate Trégard, who is in hiding. In his efforts, Michael is aided by his man-servant, César, and by two Italian friends, Paolo and Giuseppe. One owns a small restaurant and the other owns a water-taxi. However, an assortment of shady characters attempt to frustrate Michael's efforts. These shady people, led by a man named Carl Natzka, use physical intimidation, threats, disinformation and even murder, in order to force Michael to abandon his quest. Eventually, Michael finds out that Paul Trégard is actually a patriot who only pretended to be a traitor in order to infiltrate a network of foreign spies and gather evidence about their undercover agents working in France. Trégard has compiled a list of hostile agents active in France and has hidden the list in a cigarette case, placed in a religious shrine found on a small street in Venice. But the enemy agents have become suspicious of Trégard and torture him to find out the location of the list. Michael Newman and his Italian friends must race against time to find Trégard and the list before it's too late.