Under the ruse of repairing and flying downed planes, U.S. pilot Robert Trimble rescued nearly 1,000 prisoners of war, slave laborers and concentration camp survivors in Eastern Europe. (Courtesy of the Trimble Family)

In hindsight, the memory was always sitting there like a stray piece of string just waiting to be pulled.

For years, Lee Trimble listened as his father, Robert, told stories about his service in World War II — usually downplaying the dangers, and often redirecting credit to others.

But nearly a decade ago, the taciturn former pilot, at that point in his twilight years, made reference to being depressed after returning home from his service in the Soviet Union.

Lee Trimble never heard his father tell a story about the Soviet Union before. So the next time he traveled from his Haymarket home to his father’s retirement community in Pennsylvania, he decided to ask about what happened in Russia.

Words like “top secret,” “undercover agent” and “rescue” came next.

Lee Trimble of Haymarket holds a passport that his father, Robert, had used during a top-secret rescue mission in 1945. “I found out more about my father after he passed away than I ever knew about him when he was living,” Trimble said. (Victoria St. Martin/The Washington Post)

“Slowly, it came out he had been in Russia for what I call now his second war, which lasted as long as his time as a bomber pilot in England,” said Trimble, 65, while sitting in his living room. “It all came out about him going there on a secret mission” for the Office of Strategic Services, the predecessor of the CIA. “He wasn’t even told the nature of his mission until he got there.”

The base for his father’s solo mission was Poltava in central Ukraine, which was then part of the Soviet Union.

Under the ruse of repairing and flying downed planes, Trimble said, his father rescued nearly 1,000 prisoners of war, slave laborers and concentration camp survivors in Poland.

At first, he did not believe his father. But Robert Trimble pulled out a cigar box with medals he had had hidden for years. He showed his son one medal for rescuing 400 French women, a commemorative one from Russia and a passport with the words “American Consular Service” stamped across his photograph.

Lee Trimble recorded the talks with his father, who died at 90 in 2009.

“He didn’t really want to make it public; he never intended it to be,” said Trimble of his father’s clandestine mission in 1945. “So I waited until he passed away before I began to do research because I knew if I was going to be public with this at some time in the future, I had to make sure I got the story straight that he had indeed done these things.”

After years of research and cross-referencing government records — and more than a little pulling — Trimble unraveled a tale of intrigue, suspense and peril to rival a spy thriller.

Trimble compiled his research into a book, “Beyond the Call: The True Story of One World War II Pilot’s Covert Mission to Rescue POWs on the Eastern Front,” as a tribute to his late father’s mission. It was published in February.

“He kind of went from a mundane father — and I kind of feel bad describing him that way, but up until that point he was just Dad — and then now he’s this hero, he’s this humanitarian, who had done this deed for whoever he could,” Trimble said. “I found out more about my father after he passed away than I ever knew about him when he was living.”

Lee Trimble’s daughter, Rachael Trimble, found one of the first documents on the mission in a box of hundreds at the National Archives and Records Administration. She gasped when she noticed her grandfather’s signature.

Rachael Trimble, 31, who lives in Arlington, said holidays such as the Fourth of July have a deeper meaning for her today.

“Now these holidays for me mean remembering my grand­father telling his stories,” she said. “There are so many men and women [who] have these stories that are so important. And even if they don’t tell them, even if they decide to keep them to themselves, it doesn’t make their stories any less important because they weren’t shared.”