Just outside of Salzburg, Austria, photographer Richard Bryant dives into the Fintersee Lake where, without his knowledge, he is observed by a man as he pulls a heavy chest from the water to the shore. Later, in the city, American attorney William Mathison visits Bryant’s photography shop to make inquiries about a photo book of Austrian lakes purportedly commissioned by Bill’s client, publisher James Newhart. Bryant’s wife Anna provides Bill with correspondence from Eric Yates who claimed to be Newhart’s representative, and included a down payment check signed by Emil Birch for the book. After Bill departs, Anna’s brother Johann Kronsteiner receives a call from family friend Felix Zauner informing them that Bryant has been involved in a fatal accident. A short distance from the photography shop, Bill realizes that he is being followed by two men and in order to shake off his pursuers, takes a horse-drawn carriage across town. After losing the pair, however, Bill discovers a mustached man is now trailing him and so joins a tour to the Hohensalzburg castle overlooking the city. Upon evading the man, Bill and a young blonde American, Elissa Lang, strike up a conversation ending with her invitation for a drink at Bill’s hotel. Meanwhile, Johann drives out near the lake to identify Bryant’s body and notices rope burns on the corpse’s hands. Unknown to Bill, Elissa is working for Felix, who is a member of Austrian intelligence. At the hotel, Elissa secretly phones Felix and speaks to his associate, Bernard Dietrich, who was one of the men following Bill earlier. Dietrich informs Elissa about Bryant’s death and she then contacts Lev Benedescu, a KGB agent for whom she is also working as a double agent, and passes on the information about Bryant. Later that evening, Newhart, at the behest of a CIA agent known only as Chuck, calls Bill to tell him to meet Chuck the next day. Confused, Bill tells Newhart to handle the Yates’s situation himself, but Newhart reveals that Yates is dead. While Johann drives to the edge of Fintersee in the gathering dusk, Anna visits Bill to relate the news of her husband’s death. Confessing that she believes that he was murdered, Anna asks Bill if Yates was involved and Bill informs her of Yates’s death. Disturbed, Anna reveals that her husband and Yates were once in British Intelligence together, but she never trusted Yates. She confides in Bill her belief that her husband might have been trying to involve himself in spy work again. As Bill is walking Anna back home later, Elissa breaks into the Bryants' closed shop to take Bryant’s photos of the Fintersee and Yates’s correspondence. Arriving at the shop, Bill and Anna see suspicious movement and Bill chases Elissa, unaware of her identity. Escaping into an alleyway behind the shop, Elissa is followed by a man, whom she attacks and kills, then flees. Anna and Bill find the dead man whom Anna recognizes as Dietrich. At Fintersee, Johann, followed by German neo-Nazis Anton and Grell, locates the chest at the edge of a forest, then fights off the men and takes the chest to his girl friend Trudi Seidl’s cabin. Meanwhile, Bill escorts the nervous Anna to his hotel and gets her a room. The next morning, after finding a note from Anna at the shop, Johann visits her at the hotel and confesses that he took the chest after realizing that Bryant had found it in Fintersee. When Johann reveals that an Israeli agent visited him at the shop offering to purchase the chest, Anna reminds her brother that her husband and Dietrich are dead due to the chest and believes they should dispose of it. Later that day, Bill takes a tram to a hilltop to meet Chuck and is startled when a man sitting in the seat behind him is discovered to be dead. Chuck admits one of his men killed the man, who was a Polish agent working for the Communist Chinese. Chuck then explains that the CIA had been working with double agent Yates, whose real name was Emil Birch. Yates provided Bryant with the chest which holds papers containing names and information on former Nazis who could help revive the party. When Bill expresses puzzlement at the CIA’s interest, Chuck points out that the United States employs several German nationals in crucial scientific and government positions and that the revelation of their past associations could be devastating. Chuck then asks Bill to get the chest through Anna, who will also be pressured by Felix and Austrian intelligence. That afternoon, Johann returns to his cottage where Anton and Grell await and kidnap him. Bill reluctantly meets with Anna to ask about the chest and she relates that Bryant had learned about it’s location and contents from a former Nazi and hoped that he could use it to be reinstated into the intelligence community. Back at the shop, Anna receives a call from Trudi, who informs her that Johann has gone missing. Some time later, Bill agrees to drive Anna to Bryant’s burial service, but while he waits on the street, two men grab Anna and force her into their car. Anna manages to attract Bill’s attention and he follows them through the city, then maneuvers his car in front of theirs and blocks traffic, bringing the city police to his aid. While Anton and Grell torture Johann to reveal the location of the chest, Elissa meets Benedescu, who criticizes her sloppiness in allowing Bill to remain involved in the hunt for the chest. Benedescu provides Elissa with a high powered bomb with a ten minute fuse designed as a lock and orders her to use it to destroy the chest as they have acquired the list of names by other means. As night falls, Bill and Anna go to meet Trudi, but are distracted when Anna sees Felix’s car on the street. Finding him in a small shop, they tell him that Johann has disappeared, but are interrupted by Elissa’s arrival. Surprised and suspicious, Bill immediately escorts Anna out as Felix chastises Elissa for killing Dietrich and reveals that Austrian intelligence has long been aware she was a double agent involved with Yates. Elissa assures Felix she will destroy the chest, then goes to a nearby café where she confronts a group of neo-Nazis and vows to get Johann to reveal the location of the chest. The men telephone Anton and Grell, and, pretending to be Anna, Elissa convinces the weakened Johann that Trudi will be killed unless he reveals the chest’s location. At the same time, Chuck surprises Anna and Bill and joins them at Trudi’s. As they are leaving with the chest, Elissa, the Neo-Nazis and, separately, Felix arrive. While Felix and Bill interrogate the men, Elissa places the explosive on the chest which blows up less than a minute later, killing her. While Chuck takes the men into custody, Felix tells Bill he believes he knows Johann’s whereabouts. On the drive there, Bill reveals that he and Chuck switched the chest and Felix admits that his name is on the list as he became a Nazi informer during the war to save his wife from a concentration camp. At Anton and Grell’s hideout, Felix tries to negotiate with the men but is shot as Bill breaks in and saves Johann at gunpoint. A few days later, Bill prepares to depart Salzburg and Anna agrees to go with him.