In mid-August 1939, just on the brink of World War II, the astute editor of the New York Morning Globe, Mr. Powers (Harry Davenport), dispatches crime reporter John Jones, who writes under the alias “Huntley Haverstock” (Joel McCrea](/actor/joel-mccrea)), to Europe to cover the unfolding situation. His initial assignment involves interviewing a Dutch diplomat, Van Meer (Albert Bassermann), during a luncheon.
On their way to the event, Jones shares a cab with Van Meer. Despite his probing questions about the looming threat of war, Van Meer artfully avoids giving any substantial responses. At the luncheon, Jones finds himself enchanted by Carol (Laraine Day](/actor/laraine-day)), whose role remains ambiguous. Attempting to seize an opportunity, he invites her to sit at his table, claiming that the reporters present would ignore the speeches. However, a charming host, Stephen Fisher ([erbert Marshall), announces that Van Meer is ill and can't attend, prompting his daughter Carol to speak in his stead. In a misfortunate turn, Jones realizes he has unintentionally offended the very woman he has grown to admire.
Powers subsequently assigns Jones to cover Van Meer's next appearance at a gathering of the Universal Peace Party in Amsterdam. When Jones greets Van Meer outside the conference, he notices that the diplomat appears to be in a trance and fails to recognize him. Suddenly, a photographer attempting to capture Van Meer's image conceals a firearm within the camera, leading to a shocking assassination. Jones instinctively pursues the shooter and crosses paths with Carol and her friend Scott ffolliott (George Sanders), whom they join in the chase. Though they initially lose the suspect, Jones suspects he may have sought refuge inside a windmill.
While Carol and ffolliott head off to alert the authorities, Jones investigates the windmill and discovers Van Meer, heavily sedated but alive. The diplomat can only manage to express that the man shot earlier was not who he seemed. Jones narrowly escapes to bring the news of Van Meer's survival to the police, only to find that both the diplomat and his captors have vanished when they arrive back at the scene.
Later, while in his hotel room in Amsterdam, two spies posing as police attempt to abduct him. Sensing danger, Jones leaps out of the bathroom window, where he unexpectedly encounters Carol once again. Their journey continues aboard a British ship to England, during which, amidst a raging storm, Jones proposes to Carol, and she joyfully accepts. Upon arriving in England, they visit Carol’s father, where Jones spots Krug (Eduardo Ciannelli), the man he recognizes from the windmill as being involved in the assassination and kidnapping plot. The situation escalates when Fisher and Krug, realizing Jones knows too much, plot his demise. In a twist of fate, Fisher informs Jones of impending danger, offering him a bodyguard, Rowley (Edmund Gwenn). However, this apparent protector attempts to eliminate Jones during a visit to the top of Westminster Cathedral, ultimately leading to his own death.
Determined to expose Fisher as a traitor, Jones and ffolliott concoct a plan: to take Carol to Cambridge while ffolliott pretends that she has been kidnapped to extract Van Meer’s location from her father. Carol, believing she is protecting Jones, becomes suspicious when she overhears him reserving two adjoining rooms, interpreting it as a deceitful ploy for a more intimate encounter. In a rush, she leaves for home, ultimately thwarting ffolliott’s efforts.
Meanwhile, ffolliott trails Fisher to a secret hotel where Van Meer is held captive. He finds himself in a perilous situation, captured at gunpoint and brought face-to-face with the spies torturing Van Meer for details of a critical treaty clause—Clause 27—a secret that could shift the balance as war looms. In a suspense-filled struggle, ffolliott manages to escape just as Jones arrives with reinforcements, and they rescue Van Meer before Fisher makes his getaway. The recovered diplomat is swiftly taken to a hospital to recuperate.
As Britain and France declare war on Germany, Jones and ffolliott pursue the Fishers, hoping to bring them to justice. When Fisher intercepts a telegram confirming Van Meer’s survival and acknowledgment of Fisher’s treachery, he turns to Carol, seeking her loyalty while contemplating his fate. Desperate, Jones entreats Carol to rekindle their relationship, but before any resolution can come, the airplane they’re on is attacked, crashing into the ocean.
Surviving the wreck, the group clings to the floating wing of the downed aircraft. As they grapple with their dwindling chances of survival, Fisher selflessly sacrifices himself, slipping into the sea so the others may live. Eventually, an American ship comes to their rescue. However, the captain enforces communication restrictions due to American neutrality, leaving Jones, ffolliott, and Carol to relay their findings surreptitiously. Upon returning to England, Jones stands side by side with Carol, evolving into a successful war correspondent. As he captivates listeners in a live radio broadcast, he passionately describes the bombings in London, urging Americans to "keep those lights burning" as darkness envelops the studio.
