‘Van Walters never slumped. He always stood up straight. His posture was always perfect. Secret service perfect.” But on Nov. 22, 1963, nothing was perfect for Agent Walters or anyone else. President John F. Kennedy was shot and killed. We mourned as a nation and as individuals, and there were some, close to the scene, who sustained lasting psychic scars. In “Top Down,” his 21st novel, the journalist Jim Lehrer imagines the assassination’s collateral damage to one agent and his family.

Jack Gilmore, an ambitious reporter for the Dallas Tribune, has been assigned to cover the arrival and departure of President and Mrs. Kennedy at Love Field on the morning of Nov. 22. “At the behest of a rewrite man downtown,” Gilmore asks Walters whether the bubble top on the president’s limousine is going to be on or off. Kennedy himself had “insisted his limousine be open during motorcades so the American people would not feel he was a museum piece to be viewed only under glass from a distance.”

Made out of Plexiglas, the top wasn’t bulletproof. It was designed only to protect against bad weather, and the early-morning Dallas rain had ended. Gilmore listens as Walters orders the removal of the bubble top. With a backward glance, Gilmore recounts: “There was some loose speculation afterward among law enforcement people and others, which I reported, that the bubble top, if it had been there, might have prevented the assassination — or at least the death — of Kennedy.”

Five years later, Gilmore, who has “become the Tribune’s go-to reporter on the assassination,” gets a distraught phone call from Walters’s daughter Marti. “My father’s life is at stake,” she tells him. “I must speak with you — in person, as soon as possible.”

Sensing that this might be a powerful, possibly even career-boosting story, Gilmore agrees to meet with Marti, who initially insists that whatever she tells him must be “off the record — way, way off the record.” When he objects, she tearfully implores him: “Promise me first and if it ends up leading to a story you want . . . Well, we can talk about it then.” Once Gilmore concedes, she immediately blurts, “My dad believes he’s responsible for the death of John F. Kennedy.”

“Top Down: A Novel of the Kennedy Assassination” by Jim Lehrer. (Random House Publishing)

If a juicy headline dances in Gilmore’s head, he knows that Marti needs to talk, and he lets her talk for hours. Her father believes that, had he not ordered the bubble top removed, Oswald “or whoever did the shooting — might not have taken the shots.” Agent Walters is in the throes of a mental breakdown because of his overwhelming guilt, and his daughter fears for his life.

Marti hopes that whatever Gilmore knows will help her convince her dad that it wasn’t his fault. After several more meetings and a noticeable attraction between reporter and source, Marti comes up with the idea of “reenacting Oswald shooting at Kennedy with the bubble top on the car.” As she puts it, “What if we could prove to Dad that the rifle shots would have shattered that Plexiglas and killed everyone?”

A former Marine, Gilmore knows how to shoot a rifle. He also realizes he’s becoming intrigued with Marti, even though she’s his junior by a decade. She’s quirky and smart, “pretty in a no-makeup sort of way,” and, for 1968, “a kid of the times.” A student at the University of Pennsylvania, she’s interested in American literature — “the good kind,” she tells Gilmore, “mostly the kind written by women.”

Lehrer’s story is told almost exclusively in Gilmore’s voice, reporting what others say, mulling over facts and motives, making conjectures about the assassination and then retracting them, keeping the reader intrigued. There is fascinating information about the Secret Service, conspiracy theorists and the culture of the 1960s. Lehrer, one of the greatest broadcast journalists of our time, gives his go-to reporter intelligence and insight, elegance, grace and humor, charm and morality. We follow Gilmore as he observes and tries to help a family in extremis — Marti is obsessed with saving her father, who is losing his will to live while her mother is taking to drink. Well before the arrival of the psychiatrist who offers what was then cutting-edge treatment, today’s readers will recognize Walters’s condition as post-traumatic stress disorder.

Will the planned reenactment free Agent Walters from his guilt? Will Gilmore and Marti become involved? Will Gilmore get Marti’s approval to write the most important story of his career? All of these threads weave together with the right amount of suspense, and the story rushes like a thriller toward its extraordinary climax. At the end, Gilmore gives this powerful summation of the assassination’s irreversible effects: “For me, the fragility of what we all come to think of as order and normality has been the permanent lesson of the Kennedy assassination. Since that awful day we’ve known we are always only three shots away from chaos.”

Eugenia Zukerman, a flutist and the author of four books, was the arts correspondent on CBS News’s “Sunday Morning” for more than 25 years.

TOP DOWN

A Novel of the Kennedy Assassination

By Jim Lehrer

Random House. 190 pp. $26