It takes a few minutes to settle in and figure out just what the hell is going on in “Dark Journey,” a twisty and handsomely-mounted romantic thriller of the espionage variety produced on the eve of World War II.

The somewhat melodramatic World War I story is framed within a stop-and-search episode involving a Dutch coastal freighter/passenger ship and a suspiously-knowing German U-Boat crew. They’re looking for Allied spies living under cover in Sweden, one spy in particular. It’s 1918, and the German officer running the show wants to see more than just the lady’s passport.

As the tale flips back and forth, filling us in on how we got here, we see the intrigues, the wartime “neutral” trips to Paris, the cleverly-disguised maps and secret plans woven into the fabrics of French high fashion, the laborious means of decoding that and signaling, via semaphore, to offshore German agents just what the pretty “Swiss” agent has found out about French war plans.

The German aristocrat, new to Stockholm, appears to be disgraced, a “traitor to the Fatherland” but obviously on some playboy mission to Sweden to figure out the loyalties of Germany’s most beautiful agent.

Can Baron Karl Von Marwitz sell his womanizing reputation to such a degree that he will seem smitten with the fair Madeleine Goddard, dress-seller to the well-dressed and double agent extraordinaire? He’ll have to find a lot of excuses to visit Madeleine’s shop, buying dresses for his increasingly suspicious paramour (Joan Gardner).

“It used to be all girls with no clothes,” his valet gripes. “Now, it’s all clothes and no girls. Pity.”

Vivien Leigh — born Vivian Hartly in British India — was all of 24, a mercurial beauty and heartbreaker at the beginning of her big-screen career. “Gone With the Wind” wasn’t yet on her horizon. Hans Walter Conrad Veidt was 44, a screen legend immortalized in the silent classic “The Cabinet of Dr. Caligari” who’d emigrated to Britain as the Nazis came to power. He’d already made Germanic villains a specialty, and World War II would see Hollywood call for him to play a role in a “talkies” classic that rivals “Gone with the Wind” as among the most beloved films of all time — “Casablanca.”

World War I veteran Victor Saville, who’d go on to direct “Green Dolphin Street” and a late-career Errol Flynn epic “Kim,” and his British team conjure up intrigues in tony clubs and boutiques of Stockholm and Paris, in German and British intelligence headquarters, give us a glimpse of the French battlefield and settle into a confrontation on the Baltic Sea, all filmed in and around the soundstages of Buckinghamshire, UK.

Soundstaginess notwithstanding, this British production is damned impressive all around, even as we ponder the anachronistic fashions, jazzier-than-it-should-be 1918 nightclub and the peculiar pairing of Leigh with Veidt, who was twenty years her senior.

“So our pretty little dressmaker is a spy! What will people say, an officer of the Kaiser like me and a woman like you, Madeline?”

“They’ll say, the poor girl couldn’t help herself!”

The romance never really clicks for me, but it’s easy to see why this pairing was packaged. Leigh was the very embodiment of a spirited “slip of a girl,” as Madeleine is described here. And British cinema was just a couple of years past that “Women fight for Conrad Veidt” marketing campaign for the expat with the Teutonic accent and a flair for wearing monocles.

Leigh is good at suggesting the fear that’s creeping into Madeleine’s thinking as, even though we never see those who procure the “intelligence” for her elaborate double-agent schemes, we hear of their fates.

There’s a wit, sophistication and edge to this 1937 production that’s largely missing from the Hollywood fare of the early Production Code era. Even the storytelling style, that present-day sea confrontation framework, the way the narrative bounces back and forth from that, the murky ethics of it all and the daring idea of whipping up a love story involving a German spy and an Anglo-French one as the world teetered towards another world war, seems “out there” for its age.

Leigh is radiant and subtle, and Veidt suggests a softening of his Fatherland Uber Alles soldier in his performance that makes this credulous tale credible. And the script, by Lajos Biró and Arthur Wimperis, has all this inventive spycraft — dress pattern maps, semaphore communications to an anchored sailing yacht with a wireless set for signaling Berlin — that adds to the film’s air of knowing sophistication.

Hard to follow or not, strained “relationship” to sell and all, “Dark Journey” is a still a classic whose appeal reaches beyond the cult of “Vivien Leigh completists,” an espionage thriller that might have benefited from The Hitchcock Touch but manages to have its moments even without the Master of Suspense’s input.

Rating: approved

Cast: Vivien Leigh, Conrad Veidt, Joan Gardner, Anthony Bushell, Robert Newton, Philip Ray and Henry Oscar.

Credits: Directed by Victor Saville, scripted by Lajos Biró and Arthur Wimperis. A London Films production, on Roku, Amazon, etc.

Running time: 1:17