Mike Kells is assigned by the State Department to fly to Salzburg and meet his old friend Sam Carew, who will pass a top-secret document to him on a particular train platform. A passenger on the Salzburg-bound plane, Joan Ross, takes a liking to Mike and expresses a desire to see him again. However, at the set meeting site, Sam ignores him because he is apparently being tailed by two men. Mike boards the same train and sits near a woman Sam seems to know. In a tunnel, Mike is shocked to see the two men throw Carew's body off the train. Col. Cagle and Sgt. Guelvada of the US Army tersely interrogate Mike as to what went wrong. They believe the woman's involved and order him to travel to Trieste to find her. Guelvada goes along. She is identified as Janine Betki, a singer and a possible Russian agent. Mike goes to a club where she once performed. He runs into Joan there instead. After a strange man slips Mike some information, the mantries to flee for safety but is murdered in a hit-and-run incident by a car which had almost killed Mike a moment before. Janine is located and explains to Mike that she not only worked with Carew but also loved him and spied on the Russians on his behalf. Still, the colonel insists Janine was a loyal Soviet agent by showing Mike the dossier on her efforts as a Russian spy, a dossier which was compiled by Carew himself. Joan then contacts Mike and claims a sniper tried to kill her. After he leaves, it is Joan who is revealed to be the Russian agent. Mike deduces that Carew hid microfilm in a wristwatch. He retrieves the watch from the pawn shop where she left it, only to have Joan try to take it from him at gunpoint before she is surprised and overpowered by the faithful American Sergeant. Meanwhile, a complication arises that no one anticipated because the pawnbroker had cleaned the watch and removed the film. After Mike is captured by the Soviets, Janine successfully bargains for both of their lives by agreeing to give the microfilm to them. In the end, the microfilm is recovered by the American authorities while Mike manages to meet with Janine in the presence of her Soviet spymaster in a compartment on a moving train, where he ultimately facilitates their escape to freedom.
