With the word “heroic” once more in widest possible use, it’s important to remember that desperate times don’t always bring out the best in everybody.

On that note, PBS’s “Masterpiece” viewers might march right into “World on Fire,” a sweeping miniseries (premiering Sunday) about several people caught up in the opening salvos of World War II, with expectations of seeing broad displays of courage and resilience. How uplifting at this particular moment!

What you get, instead, is a chillier and occasionally provocative rumination on how hard it can be to navigate an altered world. These characters are full of ambivalence, doubt and occasional resignations to their fates. Their heroism often feels like too little, too late.

That’s history for you — messy and not always narratively compliant. Created and written by Peter Bowker, “World on Fire” opens in 1939 on Harry and Lois (Jonah Hauer-King and Julia Brown), a pretty pair of lovestruck young socialists in Manchester, England, causing a ruckus at a rally of Nazi sympathizers. Soon enough they are parted, as Harry heads to Poland to work as a translator and Lois works as a jazz singer entertaining the British troops.

Six months later, in Warsaw, Harry has fallen in love with a cafe waitress named Kasia (Zofia Wichłacz), just as her father and brother are heading off to join the fight against the Nazi invasion. Things turn awful very quickly, and Harry’s plans to get Kasia out of the country go awry; instead he returns to the English estate of his emotionally distanced mother, Robina (“Phantom Thread’s” Lesley Manville), accompanied by Kasia’s frightened little brother, Jan (Eryk Biedunkiewicz).

The first hour of “World of Fire” involves a dizzying degree of characters and plot threads — some might call it “meanwhile abuse.” Helen Hunt gives a staunch but satisfying performance as Nancy, an American radio correspondent based in Berlin, whose increasingly frantic dispatches are censored by her Nazi minders. Her nephew, Webster (Brian J. Smith), is an American doctor in Paris, who is falling in love with a French African jazz saxophonist, Albert (Parker Sawyers), just as the Germans gear up to invade France.

Back in Manchester, meanwhile, Sean Bean (“Game of Thrones”) plays Lois’s father, Douglas, a World War I veteran whose experiences have turned him into a pacifist.

I’m sure I’m leaving others out, but that’s only so I can offer the relieving news that “World on Fire” does settle into a watchable rhythm, as the connections between all these characters begin to emerge and cohere. The writing is brisk and efficient, but it does take a few more episodes (there are seven in all) to make the best use of actors as talented as Hunt, Bean and Manville.

“World on Fire” struggles most on battlefields and in other scenes requiring cinematic scope. (After all, who wants to stage Dunkirk after we’ve all seen “Dunkirk”?) What’s more interesting about “World on Fire” is what it doesn’t do: There’s never a swelling, Churchillian sense of British resolve and pride, and, amid the Holocaust, there’s practically nothing in the Warsaw segments about what happens to Jews.

Instead, “World on Fire” zooms in on other victims, such as Nancy’s German neighbors (Johannes Zeiler and Victoria Mayer) who try to hide the fact that their young daughter (Dora Zygouri) has epilepsy and is subject to the Third Reich’s horrifying roundup of the sick and disabled.

Where we expect deepening displays of sympathy and resolution, the series often veers toward the opposite. We also get one heck of a cliffhanger at the tail end. After the disappointment of being left in the lurch by “Sanditon” earlier this year, “Masterpiece” viewers can at least take comfort in the news that a second season of “World on Fire” is in the works. It would be nice to give these morose characters a fighting chance.

World on Fire (one hour) from PBS’s “Masterpiece,” premieres Sunday at 9 p.m. on WETA and MPT.