Howard Graham, an American armaments engineer working with the Turkish Navy, writes a letter to his wife Stephanie, explaining the perilous circumstances that led to their recent separation in Istanbul: While en route to the United States, the Grahams stop in Istanbul and are met by S.  Kopeikin, a Turkish employee of Graham's company, who under the pretense of discussing business, takes Graham to a nightclub.  There Graham meets the Eurasian dancer Josette Maretl and her partner Gogo.

When a bullet meant for Graham kills the nightclub's magician instead, Colonel Haki, the head of the Turkish secret police, comes to investigate.  Haki is concerned for Graham's safety because the engineer has irreplaceable knowledge about the armament needs of the Turkish Navy, and consequently, his demise would mean a delay in arming the Navy.  After showing Graham a photograph of Peter Banat, an assassin hired to kill him by the Nazi agent Muller, Haki tells Graham that he has arranged safe passage for him aboard a tramp steamer bound for Batumi.  When Graham protests being separated from Stephanie, Haki assures him that he will accompany her to Batumi.

At dockside, Kopeikin bids Graham farewell and presents him with a pistol, which Graham then hides under his mattress.  On board the ship, Graham meets his fellow passengers: Josette and Gogo; Kuvetli, a Turkish tobacco salesman; Professor Haller, an archeologist; and Madame Mathews and her socialist husband.  Lonely and frightened, Graham is befriended by Josette.  When the ship makes its first stop, Graham cables Stephanie to meet him in Batumi on Saturday.  After the ship leaves the port, Graham is alerted to the arrival of a new passenger by the musical strains of a record player, and Haller warns him that Kuvetli is not who he claims to be.

At dinner that night, the new passenger joins Graham at his table, and the engineer recognizes him as Peter Banat.  Panicked, Graham tells the ship's captain that there is an assassin on board, but the captain thinks that he is demented and laughs in his face.  Upon discovering that his gun is missing, Graham turns to Josette for help.  After listening to Graham's story, Josette offers to have Gogo detain Banat in a card game while Graham searches his cabin.  When Graham returns to his own cabin, he is met by Haller, who identifies himself as Muller.  Haller offers to spare Graham's life if the engineer will delay his return to the States for six weeks.  Explaining that the Germans seek only a postponement in the communication of Graham's recommendations for arming the Turkish Navy, Haller suggests that he check into a hospital with a case of "typhoid."  Before departing, Haller informs Graham that Kuvetli is a Turkish agent sent by Haki.

After Haller leaves, Kuvetli contacts Graham and instructs him to consent to Haller's plan.  Kuvetli also tells Graham that when the ship docks in Batumi, he should hide while the Turkish agent arranges for the arrest of Haller and Banat.  Graham follows Kuvetli's instructions and agrees to Haller's terms, but when he goes to contact the Turk, he discovers the agent has been murdered and hears the familiar strains of a musical recording.  Desperate, Graham asks Mathews to deliver a message to the Turkish counsel.  Mathews agrees and offers Graham a pocket knife and an umbrella for protection.

When the boat docks, Banat and Haller escort Graham ashore and into a waiting car.  As they are driving, one of the car's tires goes flat, and when the driver leaves his seat to examine the tire, Graham sticks the pocket knife into the horn, causing it to sound.  During the ensuing commotion, Graham jumps into the driver's seat, crashes the car into a store window and escapes his captors.  Then as a storm rages that night, Graham runs into a hotel and joins his wife in her room, but finds Haller and Banat waiting for him.  Thinking that Haller is a representative from her husband's company, Stephanie leaves them to discuss business while she joins Haki for a drink in the hotel bar.

Soon after, Gogo, who has been trying to make a deal with Graham for Josette's "services," knocks on his hotel room door.  When Gogo opens the door, Banat shoots at him, sending him scurrying to the lobby for help.  Graham uses the diversion to jump out the window onto the cornice of the building, where he is followed by Haller and Banat.  At that moment, Haki appears on the ledge and shoots Haller.  Banat then wounds Haki and turns his gun on Graham.  Blinded by the rain, Banat misses his target, and after emptying his pistol, the two men struggle and Banat falls from the ledge to his death.  Safe in the hotel, Graham finishes writing the letter to his wife that he began on board the ship, and when Haki tells him that Stephanie is waiting for him upstairs, Graham tears up the now completed letter and joins her.