I once heard a master of suspense say that the craft was actually quite simple: Take a perfectly normal situation, a trope readers know well, then throw in a wild “what if?” What if your mild-mannered, homebody spouse — so familiar to you — is the midnight stalker in the black balaclava? What if the buttoned-down banker, the one who always takes home the civic awards, is knee deep in sex and depravity? What if your president — he who died martyred and tended to be a wee sickly — was a thrill-seeking spy at a pivotal time in history?

It’s a lesson Francine Mathews seems to have learned well.

Her “Jack 1939” is most assuredly a work of fiction, but it takes skeins of history we all know well — Churchill’s England, Hitler’s Germany, Roosevelt’s White House, the rise of the Kennedy family fortunes — and ravels a hair-raising tale.

In it, John F. Kennedy is young Jack, a junior at Harvard languishing in the Mayo Clinic and eager to board the Queen Mary for a much-needed rest in England. His father, Joe Kennedy, is the ambassador to the Court of St. James; his father’s rival, Franklin Delano Roosevelt, is angling for a third term; Hitler is busily cooking up a pact with Stalin; Himmler is madly devising the Final Solution; and war is in the air. But Jack is less driven by battle drums than a broken heart. The girl of his dreams has just thrown him over, and he is off to to Europe to stanch the wounds. Maybe even write his Harvard thesis. So far, all this is true. We’re in the comfortable zone of history.

The “what if” arrives in fairly credible guise: What if FDR, who always wanted a leg up on J. Edgar Hoover, decided to run his own reconnaissance missions in Europe? The president believes that Hitler is up to no good, that world war is inevitable, and he aims to put his own man on the ground to prove it. Enter Jack, ambling up Park Avenue to the Waldorf Astoria, the day before he embarks on the trip of his life.

’Jack 1939: A Novel’ by Francine Mathews (Riverhead. 361 pp. $26.95). (Riverhead)

“ ‘Mr. Kennedy?’ ”

“There were three of them. . . . They wore trench coats and snap-brim fedoras, and although they bore no relation to one another, their faces had a blunt-featured sameness. . . . ‘We’re President Roosevelt’s bodyguards. He wants to see you.’ ”

Jack is hustled off to a secret meeting on the president’s Pullman train, which awaits on a private track under the bowels of the Waldorf. It turns out FDR has been keeping an eye on him for some time, figuring that a smart boy with a failing body will have little to lose in a risky bargain. FDR gives him a mission, a special code, a telegraph machine, and sends him out to learn what he can. “They’ll never expect you to be my man in Europe,” the great man tells him.

I know, I know. It’s an iffy “what if.” But before we can say, “Whatever happened to Wild Bill Donovan?,” we’re swept into the conceit, hoping for a diabolical labyrinth worthy of le Carre; a cunning caper reminiscent of Ludlum. What we find instead is a cross between Jane Austen and Dashiell Hammett.

“And then she materialized beside him: cool and porcelain-faced, knees bound in a pencil skirt. Her fur was high-collared and ended abruptly at the waist. Her hat swept like a dove’s wing over one cheek. It made her seem sly and seductive and unreachable as she stared thoughtfully at the pier. Where was her farewell party? Like Jack, she did not bother to wave. Like Jack, she crossed her arms and leaned on the rail, one shoulder grazing his. Her mouth was painted crimson. An unlit cigarette dangled from her lip.” Mind you, the Hammett-Austen routine is no coincidence. Mathews has cut her teeth on a drove of mysteries that purposefully conjure Dashiell and Jane.

In any case, it is there on the deck of the Queen Mary, as fate would have it, that Jack encounters the angel and villain of the story. The angel is the beauty whose cigarette is waiting to be lit: Diana Playfair, a married woman with a cloche of black hair and a reputation for siding with Germans. The villain is a Nazi with a mean knife, a killer who has slashed his way through Hell’s Kitchen, knows Roosevelt’s secrets, and is cutting a quick course for Jack Kennedy’s gut.

Plausible? It doesn’t much matter. The pace is so propulsive that you’ll read every word.

Mathews is no newcomer to the suspense story. She is a former CIA intelligence analyst and the author of more than 20books. As Francine Mathews, she has written the Merry Folger mysteries, cliffhangers set in present-day New England. As Stephanie Barron (her middle and maiden names), she has written 18th-century thrillers with Jane Austen as her star sleuth.

With “Jack 1939,” she steps into entirely new territory.

Mathews’s ability to weave fact into her tale is nothing short of remarkable. Here is Joe Kennedy, grooming himself for the presidency; Rose, flouncing her way through Egypt; Missy LeHand, shuffling in and out of the president’s bedroom in slippers and robe. Here is Hitler’s “man with the iron heart,” Reinhard Heydrich, whose Enigma machine will be crucial to war intelligence; and here, too, are heroes of “irregular warfare,” among them the dashing Englishman Colin Gubbins, with his bright eyes and smiling mustache. Young JFK actually did travel to Prague, Warsaw, Moscow and Vienna, returning to London the day Germany invaded Poland. Indeed, a photograph of him juggling oranges in Nuremberg inspired this novel in the first place. What’s more, he wrote his senior thesis on the blunders of pre-war Europe and published it a scant few monthslater as “Why England Slept,” with an introduction by none other than Henry Luce.

But in this caper, young Jack crisscrosses Europe, chasing as much as he’s chased, falling in and out of bed with Diana, sending coded messages to FDR from hotel rooftops, and all the while, the shadow that will most likely kill him is his Mystery Disease. To prevent it, he must stuff tiny steroid pellets into self-inflicted cuts in his legs. Did you ever wonder at that moon face? Well, it is true that JFK took steroids (and a whole pharmacy of painkillers) for a good portion of his life. Mathews has done her research.

There are, to be sure, imperfections in “Jack 1939”: the unfortunate cliche, the too-obvious harbinger of doom, the inclination to describe characters in reductive form: a woman is elegant via Chanel, rather than anything so human as a gesture or mannerism.

But let’s not be peevish. It’s going to be a long, hot summer in Washington, and there are precious few entertainments this captivating. What if . . . you jump off that train somewhere between Budapest and Bratislava and roll to freedom, clutching a world-class secret in your hands?

Arana, a writer at large for The Washington Post, is a former editor of Book World. She is the author of “Cellophane,” “Lima Nights” and a forthcoming biography of Simon Bolivar.

Ron Charles is on vacation.

Jack 1939

By Francine Mathews

Riverhead. 361 pp. $26.95