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On April 9, 1950, the communications section of the Department of State receives an urgent message and immediately requests a diplomatic courier to undertake a secret assignment.  The courier chosen is Mike Kells, who is instructed to meet his old Navy pal, Sam Carew, at the train station in Salzburg.  There, Sam, who works in the Bucharest legation, is to give Mike a document to transport to Washington, D.C.

When he boards a plane bound for Salzburg, the exhausted Mike falls asleep on the shoulder of Joan Ross, an American widow whose late husband also worked for the State Department.  When Mike awakens, the seductive Joan flirts with him and gives him a lift to the train station.  Mike regretfully states that he cannot pursue a relationship with Joan, although she promises to look him up.

At the station, Mike is baffled when Sam ignores him, but observes that he is being followed by two thugs.  Uncertain what to do, Mike follows Sam onto the train, and sits in the dining car with a lovely Czech woman.  Mike then sees Sam hurry into the woman's compartment, but when he tries to approach Sam soon after, Sam shakes his head and disappears.  Mike searches for him, and when the train goes through a tunnel, is shocked to see the two thugs toss Sam off the train.

Mike takes Sam's mutilated body to a nearby military base and there, Colonel Mark Cagle, of the criminal investigation division, and his aide, Sergeant Ernie Guelvada, interrogate Mike, who admits that he did not receive Sam's document.  Cagle wonders if the woman Mike spotted is involved and arranges for him to go to Trieste, so that he can identify her when she disembarks.  Ernie reprimands Cagle for not telling Mike that he is being used as bait for the Russian agents who were seeking Sam's information, but Cagle dismisses his concerns.

Ernie then accompanies Mike to Trieste, which, since the end of World War II, has been swarming with secret agents on all sides of the Cold War.  Using several clues, Mike learns that the woman's name is Janine Betki, and when he questions the local military police, they inform him that Janine slipped off the train outside of Trieste during a routine passport check.  Wondering if Janine, an entertainer, would try to get a job, Mike goes to a nearby nightclub, and there runs into Joan, who confesses that she used an embassy contact to find him.

While Joan bids farewell to some friends, Mike is approached by a man who offers him Sam's watch, which Mike recognizes immediately, and gives him Janine's address.  When Mike tries to question the man further, he runs from the nightclub and is hit by a car, which narrowly misses Mike.  Although he believes that Janine set him up to be killed, Mike goes to her apartment, where she is bewildered by his hostility.  Janine reveals that she and Sam were in love, and that she had worked as a double agent for him while he was trying to get her to America.  Janine also explains that Sam was aware of the thugs on the train, which is why he did not acknowledge Mike, and that he must have given Mike the clues that led to his identification of her.  Mike doubts Janine's story, but when she recounts several facts that only Sam could have known, Mike is convinced that she is telling the truth.

Their conversation is interrupted by a pounding at the door, and after Mike leaves, he is beaten by two men.  Ernie rescues Mike and takes him to Cagle, who refutes Janine's story by asserting that she is a well-known Russian agent who was placed on the suspect list by Sam himself.  The bewildered Mike insists on knowing what Sam was trying to smuggle to Washington, and Cagle reveals that it was the "complete Communist timetable," including information about an invasion of Yugoslavia.

Later, Joan insists that Mike come to her hotel, where she tells him that a sniper shot into her room and just missed her.  Suspecting that Mike has gotten her involved in a dangerous matter, Joan demands the truth, but Mike evades her questions.  After Mike leaves, Rasumny Platov, the head of the Soviet Secret Police in Europe, visits Joan, who bitterly states that she has never had such trouble insinuating herself with a target.

Outside the hotel, Janine finds Mike and tells him that she now knows where the document is, and that he must meet her at her apartment that night.  Mike reluctantly goes to her flat, where Janine tells him that she wants safe passage to America in exchange for the document.  Mike agrees but Janine is distracted by a phone call she believes is from Sam, who apparently is not dead.  Mike mistakenly thinks that she is trying to set him up and storms out, then deduces that the document must have been a piece of microfilm hidden in Sam's watch.

Retracing Janine's steps, Mike locates the watch in a repair shop but leaves before the owner can tell him that he cleaned the watch and found something inside it.  Mike and Ernie race to his hotel, but as Mike begins to open the watch, Joan enters the room and, leveling a pistol at Mike, demands the watch. Joan then tells him that she has been a foreign spy for many years, but Ernie manages to disarm her.  Mike then receives a phone call from Cagle, ordering him to meet at a deserted square, but the call is actually from entertainer Maximillian, a Soviet agent who does impersonations.  Maximillian was also behind the call to Janine and has lured her to Platov's headquarters.  There, Janine tells Platov that she has the document and will give it to him if she is given her freedom.  Platov agrees, but before she goes, Janine is able to leave a message for Mike, who is rescued after Platov's men dump him in the harbor.  Janine's message enables Cagle to recover the microfilm, but when he states that they cannot help Janine, Mike deduces which train she would be on and finds her.  Janine is guarded by Platov, but Mike succeeds in disarming him and climbing out the window with Janine.  After the couple roll down a hillside, Janine tells Mike how grateful she is to be free, and he comments on how lovely she looks now that she is "just a girl," rather than the enemy.