When a WWII spy (Marius Goring) is caught at Holyhead transporting information to the enemy in neutral territory—"The Irish Free State"—Captain David Grant (Robert Newton) of British Military Intelligence leads the hunt to find missing scientist Nils Erich Hansen, who, according to the intercepted papers, is making strides in developing an atomic bomb for the Nazis.

Immediately hopping the titular vessel, Grant and another officer, Captain Robert Wilson (Lawrence O'Madden), set off to find Keitel (Herbert Lom), the man who was meant to receive the documents. In the ship's dining room, Wilson encounters an old acquaintance, Paul Faber (Raymond Lovell). After Wilson takes his leave, Faber pays his check, also passing the steward a folded magazine on which he finds the handwritten words "James O'Connor, New Eastern Hotel." This turns out to be Faber's assumed identity, under which he picks up the rolled magazine at his Dublin hotel, then has it delivered to Keitel in Room 70.

As Keitel's door closes, Captain Grant approaches the porter and asks where he can get another copy of the magazine he saw delivered. The boy doesn't know but suggests he ask the man from Room 48 who gave him the magazine where he got it. Grant returns to his room, which happens to be next to Keitel's and gives Wilson O'Connor's room number. Out on the balcony, Grant watches through the window as Keitel removes some papers from the magazine. As he is doing so, Keitel receives a phone call from Faber.

Meanwhile, Wilson tricks the desk clerk into confirming that 48 is "O'Connor's" room number, then gets a different desk clerk to give him the key for that room. While O'Connor is out, Wilson searches the room and discovers the occupant's true identity, but Faber learns of the deception.

While Keitel runs a bath and paces in the bathroom waiting for some soap to be brought up, Grant enters through the window, but Keitel emerges again aiming a gun at him. Grant appears cornered, but there is a knock at the door—it's the maid with the soap. She departs, having created enough of a diversion for Grant to level his own gun at the unprepared Keitel and escape through the door, taking Keitel's gun and papers, thus removing him from the picture (in both the figurative and literal senses).

On the voyage back to England that night, Wilson meets Faber for another drink. When Wilson takes his leave to get some air on deck, Leggett the steward brings Faber a note informing him that his "friend" Wilson is actually one of the detectives involved in Jannings's capture. It dawns on him that Wilson is also the man who searched his room and follows him out on deck.

Back at government headquarters the next morning, Captain Toby Hunter (Guy Middleton) is informed that Wilson's body has been found in the Irish Sea with a badly cracked skull. Among his effects, a ticket is discovered proving that he took the night boat.

Returning that night, David Grant is called in and conjectures (as though he hadn't accompanied him to Dublin) that Wilson must have followed O'Connor aboard the boat last night. Meanwhile, David's done some research of his own and discovered that James O'Connor is a phony name and puts forth his theory that O'Connor is really Faber, expecting the passenger list to confirm that he disembarked at Holyhead last night. On the contrary, the records show that a man named Paul Faber departed for Dublin on Monday and arrived back in England tonight. David is stumped but undeterred in his search for Hansen.

George Leggett meanwhile returns to an empty house, just before his wife Lily returns from a date with another man. Defiantly, she admits to the affair and claims a right to go on seeing the rich man who spoils her because George still hasn't bought her that fur coat he promised her.

Lying awake in the middle of the night, David has an epiphany. He awakens his roommate Toby to inform him that the reason Faber didn't land at Holyhead the previous night as he should have if he had sailed with Wilson is that a crew member must have helped him to hide on board until tonight. The two head down to Houston Station to find what David suspects will be Faber's distinctively punched ticket for the night boat. They find the ticket, along with a full report on Faber, from which Grant learns that Faber runs a consulting business and is presently short one clerk, who was recruited last week to join the armed forces and has not yet been replaced. Grant decides to apply for the position. He orders all other applications to be intercepted before reaching the firm. Just then the phone rings, and Inspector Martin informs David that a crew member from the night boat has suddenly come into a large sum of money. George Leggett changed £50 in Irish notes at the bank this morning, then promptly spent 30 of it on a woman's fur coat, making him the prime suspect as the person who's been "juggling with the passengers." David orders that Leggett be watched.

Faber is visited at his office by Marion Decker. She is shown in and they begin conversing in German. David then enters the outer office leaning on a cane; presents a letter of introduction to the chief clerk, Alfred Bowman; and is told to wait. As Marion exits Faber's office, she and David exchange glances.

Sitting down with Faber, David goes over his qualifications. He claims to have been employed at a firm in Newcastle for nine years until he and his partner were forced to dissolve the firm and join the armed forces; however, he has been released as being "medically unfit." Faber offers him the job, then notes that David is residing at a hotel, to which David responds that he will be looking at two apartments this afternoon. He feigns humility on being told that he "couldn't have had a better reference if he'd written it himself" and surprise at being informed that he was the only applicant for the job. On looking at two potential apartments, he is further informed by the landlady that the two gentlemen who had occupied them until just today were unexpectedly called up to join the forces. Both were taken by surprise, but David is not.

As David is settling into his new job, Toby shows up at the office asking about the open job position, but Bowman informs him it is filled. As David emerges from Faber's office, Toby greets him by the name "Henslow" with mock surprise to which David responds nervously. To the stares of the other employees, Toby remarks on how well David has done since he left the army, then exits. David awkwardly returns to his desk and prepares to leave, but Bowman summons him into Faber's office.

David asks Faber to accept his immediate resignation. Faber informs him of his suspicion that David is really a deserter from the Army, and David finds he is locked in the office, with Faber threatening to call the authorities. David begs him not to, explaining that he had "cleared off" to avoid being arrested after being falsely accused of stealing some military documents. He deftly but unconvincingly denies having sold the documents, leading Faber to surmise that David is now in financial straits. Faber offers to make a deal with him in return for not reporting him to the authorities: He has a foreign client in need of obtaining British citizenship, and if David agrees to marry her (to be divorced later), Faber will pay him £40—and give him a lucrative new job. David resists the idea of the marriage but, after a few coaxing words, gives in and signs some papers, revealing that his name really is Grant after all, not Henslow. On his way out, he bumps (literally but not unhappily) into Marion again on her way in.

Back in his office, speaking in German, Faber informs Marion that he has news for her; he believes "it" can be done. Then he asks for the ring she has brought and examines it.

Meeting up with Toby again, David lightheartedly discusses his marriage plans, saying he has no idea whom he is to marry. There is a knock at the door and Inspector Longhurst is escorted in. He informs David that he went to Faber's office looking to arrest him as "Henslow" and that Faber covered up for him completely, even though the inspector let it drop that David was wanted on a serious charge of espionage. Longhurst also presents him with an envelope containing a £50 note intercepted at Holyhead and addressed to Leggett, apparently as payment for hiding Faber on the boat. Toby needles David about his upcoming marriage in front of the inspector.

Arriving at a hotel lobby on Friday, as arranged, for his wedding, David spots the shy but lovely Marion again, and they exchange coy glances once more. Faber arrives and tells David how he thwarted the attempted arrest, apologizes that he won't be able to attend the wedding, and instructs David to meet him about "the other business" at 11:30 tonight, admonishing him not to discuss anything about it with his new bride. David and Marion are awkwardly surprised when Faber introduces to them to each other as the couple to be married, and they spend an uncomfortable taxi ride trying to make conversation. They arrive at the Registrar's office and tie the knot.

Back at the office, Bowman informs Faber that George Leggett is on the line. Calling from a phone booth, Leggett demands the other £50 he is due, promising trouble if it isn't delivered.

Back at official headquarters, Toby continues to tease David about his marriage. David informs him that Marion is an Austrian living at a refugee hostel in Pimlico and asks Toby to have Steiner check up on her. In the meanwhile, he tells his friend, Faber wants to see him for another "job" tonight. They joke some more about marriage, focusing this time on Toby's. David teases him that at least his wife was repeating "No, Frank" in her sleep last night instead of "Yes."

David returns home and finds Marion asleep, still wearing her coat, in his living room chair. He is taken aback by her presence, but she explains that she got his address from the marriage certificate and the landlady let her in to wait for him. She knows that they weren't supposed to meet anymore, but she needs his help. She's had to leave the hostel but has no money and no place to go. She can't go to Faber, and she is running from the police.

There is a knock at the door. The landlady checks to see that it was all right to let the girl in. When she leaves, David offers to take Marion to a place where she can tell him all about it in a "more congenial atmosphere"; it might cheer her up. Wary about being seen, she agrees to go along with him to a nightclub.

Marion tells David how, on the death of her parents and the departure of her brother for London, she ended up living with her uncle, a "hard," "ruthless," "fanatical" Nazi—"the most feared and hated man in Vienna," Rudolf Decker. Because of him, she found herself shunned by her old friends. Learning that a beloved friend of her father's was about to be arrested, she warned him of the danger, and he got away. When her uncle learned of her betrayal, she fled to London, intending to look for her brother. Once there, she avoided any mention of her association with the Nazis to authorities for fear she would be taken for a spy. Because Decker is a common name in Austria, they did not suspect, giving her a room in the refugee hostel while they "made inquiries."

The next day she went to see Faber, who had arranged for her brother to come to England, but learned that he had been executed as a spy two weeks ago, under the name of Frederick Jannings. Faber feared that his connection with Jannings would be discovered due to his contact with Marion. His solution was to have her "lose the somewhat dangerous name of Decker" through marriage to an Englishman and for her never to come to him for help again.

However, back at the hostel tonight Marion became alarmed by the presence of Steiner, an old acquaintance "who was once chief of the Viennese police," "forced to leave Austria" because of her uncle. Fearing she would be recognized and deported back to Austria, she came to David, the only other person she knew, for help. The ring she gave Faber on her first visit to his office was sold to pay David off for marrying her.

The orchestra begins to play, and David offers to let Marion stay in the vacant room in his apartment until he can figure something out. He shows her to the room, obtaining an extra key for her from the landlady, who silently gives them a scowl of disapproval on her way out. David leaves the room, and Marion locks herself in.

At Faber's headquarters the next day, David refuses to reveal to whom he sold the alleged stolen military documents. To further test his mettle, Faber gives him his first assignment: He is to drive Bowman in a taxi to "collect something" at 12:15 that night.

As scheduled, David drives Bowman to a factory, where the latter sneaks in and takes a package. David secretly watches and follows him, sneaking up on and knocking out the security guard when Bowman is caught. The alarm goes off, and the two run away, chased by the other staff. They return to Faber with the package, but Faber notices that David has been shot in the arm. Faber tells him to go home and lay low.

David secretly calls Toby, telling him to keep the incident from being investigated so that Faber won't suspect and informing him that the package held glass vacuum containers, likely filled with a radioactive chemical. Toby cheerily tells him Steiner is there in his office and has discovered that David has married "smack into the middle of the Gestapo"; his new wife is the niece of Rudolf Decker. She's run off from the hostel, and they can't find her. "That's all right, old boy," David tells him, "She's at my place, in bed," and hangs up.

Back home, David knocks on Marion's door. He apologizes for the intrusion, but he's lost his key and needs to borrow hers to get into his room. Letting him in, she notices he is hurt and, helping him off with his coat, she sees the bullet wound. Rolling up his sleeve, she bandages his arm. She tells him she trusts him, yet he won't reciprocate her honesty about herself by telling her what he does for a living "when ... not marrying for money." He thanks her and goes to his room.

The next morning, Faber reads with satisfaction in the paper, "Watchman attacked, unable to describe assailants." Then he unfolds a small piece of paper delivered to him and reads it.

David clumsily prepares a breakfast of burnt toast and coffee, and he and Marion sit down to eat it together.

Mrs. Hughes shows Mr. Bowman into Faber's breakfast nook, and Faber dismisses her, informing her he's going away for the weekend. He assures Bowman that they have not been discovered and it's safe to send Grant to Ireland.

David tells Marion to stay in her room while he's out, just to be safe. But first he's going to "slip round for a paper" to give her something to pass the time while he's away, and she offers to clean up the table. As he leaves, he tells her to open the door for him on his return; his key is still missing. She watches fondly out the window as he walks down the street.

Bowman is admitted to the building and knocks at the apartment door. Thinking it is David, Marion opens it to their mutual shock. Bowman demands to know where Grant is. The phone rings, but just then David arrives in the foyer below and takes the call there. It's Toby with the news that Leggett is on his way from Holyhead to see Faber, and David promises to handle it. Then Toby tells him the plan. Unbeknownst to them, Bowman is listening to the whole conversation on the upstairs extension.

There's another knock on the apartment door. Bowman hides behind it with his gun aimed at her while Marion answers it. It's the landlady with some newspapers for her; Mr. Grant had to go out again, she tells her.

David goes to the office but sneaks in through the back door, while Leggett shows up in Faber's office demanding the other £50 for hiding him on his boat. Both are unaware that David is listening at the door. Leggett threatens to report the "foreigner we took down to Devon." Coolly, Faber offers him £100 to accompany him to Devon again this afternoon, telling him where and when to meet. But Leggett refuses, demanding the full £100 anyway. Faber tells him to wait in the other room (where David has been hiding) while he gets the money from the bank. David sneaks out the back again, and Bowman arrives through the front, breaking the news to Faber that "Grant is military intelligence"—and the only person currently watching them.

Meanwhile, the unwitting David calls Toby to inform him that he's about to be led straight to Hansen, who's hiding out in Devon, and gives him the rendezvous info so that Toby can arrest Faber when the 2:50 train reaches the station; he believes Faber means to take David with him.

Faber's phone rings; it's David, letting him know that, "If you want to see me, I'm not far away." Faber tells him to meet him at his flat, not the office. He plans to take David to Devon with him, and Bowman, thinking David is still unaware of the plan and that no one will know where he's gone.

David shows up at the flat and is invited along on the expedition; however, to his well-concealed surprise, they'll be leaving this morning on the 10:50 train.

The 2:50 train arrives at the station and departs again; neither Grant nor Faber gets off. Toby calls headquarters. Finding no word of him there, he rings David's apartment. The landlady tries to ring the call through, but Marion lays unconscious on the floor. At last she hears the ringing and picks up the phone but collapses again. Toby realizes something is wrong, and the officers converge on the apartment to question Marion. She reports her encounter with Bowman, and they figure out that Bowman overheard the conversation between David and Toby and that David doesn't know his cover is blown. Marion is worried about David, but Toby reassures her and tells her to rest as the officers take off. They search Faber's office and, going out the back door, they find Leggett's body in the taxi parked there.

In Faber's flat, they find the telegram he was reading that morning over breakfast. It turns out to have been sent from the town of Hunstable in Devon, narrowing down Faber's destination. But as they prepare to head out, Toby realizes none of them has ever seen Faber. They resolve to take Marion along to identify him.

Faber, David, and Bowman arrive at a stately mansion and are informed it was once a monastery. They are introduced to their hostess, Sidney Vane.

The detectives arrive in Devon, where they are informed that the sender of the telegram could not be traced; it was sent from a call box. Inspector Emerson has gone on to Hunstable, where he will meet them.

Miss Vane leaves the three visitors to talk in private, when a man emerges from a side door: Professor Hansen. He greets Faber and Bowman as "Mr. Sales" and "Hendrick." They introduce him to a thoughtful-looking David Grant.

At the station in Hunstable, the detectives question the officers there. None of them can recall Faber and company getting off the train. However, with his keen eye, Toby recognizes the pastoral scene in a travel poster for Hunstable on the wall—it's a reproduction of the original painting he saw over the mantle in Faber's flat. The local officers inform him that it was painted by Sidney Vane, a popular local artist. The detectives head for her house. They are warned on their way out that Miss Vane knows a lot of important people, but so, retorts Toby, does he.

Back at the mansion, Faber tells Miss Vane that they'll have to host Grant for dinner; there's nothing to fear; he's unarmed. After dinner, Hansen excuses himself, and Faber offers him the "containers" he's brought—Miss Vane goes to help him find them in Faber's bag. Still in the dining room, David comments on Hansen's continuing belief that he's working for the British government. Faber informs him that the servants are kept away from the scientist and adds suggestively, "Hood never talks. Do you Hood?" Hood is duly silent.

Bowman stands up and aims a gun at David, saying, "We know who you are." But David coolly deflects him with, "Mind that doesn't go off. What would Dr. Hansen think?" Faber apologizes for Bowman's rudeness, but David remains at gunpoint as the other three direct him towards the door. Faber promises to make apologies for him to Dr. Hansen for his "sudden departure."

The doorbell rings and Hood is sent down to the laboratory to keep a surreptitious eye on Dr. Hansen while Miss Vane goes to answer. It's the local police inspector with a few questions for her, along with Marion and "Captain Hunter of Military Intelligence." Vane pretends never to have heard of Faber; she says he must have bought her painting through a dealer. Suddenly Marion speaks up: she recognizes the ring Vane is wearing as the one she originally gave to Faber.

Faber and Bowman enter pointing a gun, and Vane, caught in her lie, warns her cohorts that "there are more of them outside." Faber has Bowman lock them in the house, then sends him downstairs to bring Hansen to the exit. But reaching the darkened catacombs below, Bowman is accosted and knocked out by David.

Faber and Vane exit the room, locking Marion, Toby, and the inspector in. Toby opens the window but finds himself looking out over a dangerous precipice. Undaunted, he climbs onto the ledge and begins to scale the side of the house.

Down in the basement, Hood spots David and pulls out his gun.

As the inspector and Marion watch anxiously, Toby makes his way along the side of the house and climbs in through another window.

David continues down the passageway and reaches an exterior opening, unaware that Hood has been trailing him. He spots Faber and Vane down below getting into a boat. He turns around just as Hood approaches and catches his arm. They struggle, and Hood falls to the rocks below. His scream attracts the attention of the pair in the boat, and they walk over to examine Hood's body. From the shadows, David leaps down upon Faber, attacking him from behind, discharges Faber's gun into the air, and disarms him. While the two men fight, Vane picks up the gun. David prevails over Faber, knocking him unconscious, but just as Vane points the gun at him, Toby appears, ordering her to drop it.

At last the authorities free Dr. Hansen and get the key from Faber before he and Vane are led off to jail.

Toby hands David the key and informs him that his wife is in the other room looking for him. But as he walks through the door, an alarmed Marion smashes him over the head with a vase, knocking him to the floor. "Oh, David," she exclaims, realizing who he is, and rushes over to him, where he lies in a daze comically waving his finger as though conducting an invisible orchestra.

"By deceiving a hotel porter, a plain-clothes detective named Wilson uncovers a suspect, Mr. O'Connor. This is the alias by which Faber, the head of the smuggling ring, goes. Shortly afterward, Wilson is found in the Irish Sea with a smashed skull. Later, it is learned that the ship's steward, Leggett, is suddenly in possession of a large sum of money. The insatiable demands of his fashionable wife drove him to smuggle the drugs aboard and to hide Faber on the ship.

"Detective David consults in his office with a military-service clerk. [Translation of previous sentence is questionable due to missing text deleted by an unfortunately positioned punchhole.] It is imperative that they find a man named Hansen, in whose hands the threads of the extensive cocaine-smuggling ring seem to intersect.

"Marion Decker, a distant relative of Faber from Vienna, has followed a rich man to England, who then left her in a lurch without any means of support. Faber wants to involve Marion in industrial espionage work. However, for this purpose, she must be an English citizen, which can only happen by immediate marriage to an Englishman. Faber convinces the clerk whom he has hired, David, through an offer of a considerable amount of money, to marry Marion on paper. David lets on that he is wanted as a military deserter, due to his being suspected in the disappearance of some secret papers, and that he is now in financial straits. Faber increases his incentive by proming to involve him in a special business deal.

David, Marion, and Faber

"David takes part in a drug-smuggling run posing as a taxi driver and, in the process, is shot. Marion, who has put her trust in him, bandages him. Faber wants to take David to Devon with him to see Hansen. Through a telephone conversation overheard by an accomplice of Faber, however, Hansen is warned that David is a leader in the criminal investigation department. At the house of a painter, Sidney Vane, Faber and David meet with Hansen.

"Thanks to a painting by this same artist that was found in Faber's London apartment, the detectives, with Marion's help, arrive at the secret headquarters of the smuggling ring and capture them. Marion and David meet again and it seems the sham marriage will lead to the real thing."