Robert de La Rochefoucauld had a lot to lose when he enlisted as a clandestine operative for a brand-new British spy outfit during World War II. The man could trace his family’s history to the year 900. One ancestor served as a duke in Louis XVI’s court; his claim to fame was waking up the king during the storming of the Bastille in 1789. Another wrote a book of maxims that influenced Nietzsche and Voltaire. A third, a friend of Benjamin Franklin, helped lead the movement to abolish slavery in France. Others earned renown battling in the Crusades or the Hundred Years’ War. Naturally, La Rochefoucauld grew up in a 47-room chateau about 90 minutes northeast of Paris, a place his ancestors had purchased from the daughter of one of Napolean’s generals.

But when the Nazis invaded France in 1940, the then-16-year-old wondered how he would contribute to the La Rochefoucauld legacy. How would he live up to his family’s history – and yet not allow his patrician upbringing to lull him into contentment as the nation his family helped influence convulsed under Hitler’s occupation?

Paul Kix, an editor at ESPN magazine, answers those questions with his thrilling book, “The Saboteur: The Aristocrat Who Became France’s Most Daring Anti-Nazi Commando,” a narrative reconstruction of La Rochefoucauld’s life as an operative with the Special Operations Executive (S.O.E.), a British outfit that aided the French resistance.

The book is gripping for two reasons. La Rochefoucauld cut an extraordinarily plucky path in Nazi-occupied France, parachuting into danger zones, planting explosives at Nazi factories and other facilities, getting arrested, nearly getting executed, once dressing as a nun to disguise himself, getting arrested again, and yes, killing Nazis. When you turn the pages of “The Saboteur,” you can’t help but be astonished by this man’s fortune and derring-do. La Rochefoucauld, who briefly met Adolf Hitler while attending boarding school in Germany and then later Charles De Gaulle as an aspiring commando, is ready-made for an espionage movie treatment. (Deadline Hollywood reported in 2013 that DreamWorks acquired Kix’s proposal for the book when it was called “Noble Assassin.”)

Thankfully, La Rochefoucauld earned a biographer worthy of his improbable life. Even though we know from the book’s prologue that the French aristocrat winds up living a long life, Kix builds narrative tension with masterfully detailed scenes and cliffhanger endings for each chapter.

Kix is also a gifted reporter. His main character passed away five years ago at the age of 88 – on, of all days, May 8, V-E Day – but Kix persuaded his relatives to talk. More important, according to the book’s epilogue and “Notes” section, Kix availed himself of hours of interviews La Rochefoucauld had given to an audio-recording service, plus the man’s 2002 memoir. He also combed the records of the French military and certain resistance groups – sometimes finding errors in La Rochefoucauld’s own book – and traveled extensively throughout France so he could re-create with accuracy the harrowing, often ludicrous acts of the saboteur’s life.

The book excels most during its immersive moments. One of the very best sequences happens midway through the book, in the summer of 1943, when La Rochefoucauld, by now having trained in Britain as an S.O.E. operative, makes his first mission into Nazi France to help the resistance. But as the Nazis crack down, the movement weakens, and by the latter portion of the year, La Rochefoucauld retreats and hides in a barn in a town in central France before the Nazis find and arrest him.

But it’s here where Kix’s lack of an interview with La Rochefoucauld hurts his ability to tell the story as well as possible. In prison, the saboteur is routinely interrogated and presumably tortured. But La Rochefoucauld hardly spoke about those moments in his memoir, to his relatives or in the audio interviews. “It is likely a reflection of La Rochefoucauld’s upbringing, and his parents’ dictum to never cry, complain, or even discuss one’s suffering, that Robert said little of what happened in those sessions,” Kix writes.

To Kix’s credit, he does describe some of the Nazis’ more gruesome tactics used on other prisoners – slashing the soles of their feet and forcing them to walk on salt, or soaking pieces of wool in gasoline, sticking them between a prisoner’s fingers or toes, and setting the fabric on fire. But hearing La Rochefoucauld’s recollections of the brutalities he personally faced under arrest would have given us a truer sense of the odds he overcame to escape imprisonment and survive the war – let alone become one of its heroes who lived into his 80s.

Of course, this omission is no fault of Kix’s. But the more intriguing his book became – thanks to his graceful writing and the adrenaline-pulsing plot – the more these omissions felt noticeable. I kept wondering: La Rochefoucauld died only five years ago. Had Kix just missed him? Or did he start the project while the saboteur was still alive, but perhaps not well enough or simply resistant to the inquiries of a journalist? It would have been intriguing to hear Kix discuss the backstory.

His distancing is admirable, but I kept longing for unintrusive paragraphs and sentences that would have shed light on his mission to excavate and make sense of his discoveries or dead-ends.

For instance, although the meat of the book mostly tracks the early 1940s, the prologue and epilogue focus on La Rochefoucauld’s controversial and seemingly unlikely testimony in February 1998 on behalf of Maurice Papon, a Vichy official facing charges of deporting French Jews to concentration camps. He advocated for Papon because he had known decades earlier that Papon tipped off many resistance fighters in advance of German raids. La Rochefoucauld felt a bond with him. But how exactly did Papon’s defense team recruit the famous saboteur, who by this point had been awarded France’s Legion of Honor? Kix says his family was “furious” with his testimony, and I wish we could have heard their voices to explain why.

These questions, though, surface for only one reason: “The Saboteur” is completely engrossing and elegantly told, which means any reader of this work will inevitably want more.