Mark Harmon Shares the Untold Story of a Japanese American WWII Spy: ‘No One Knew’ (Exclusive)

Mark Harmon has said goodbye to the CBS hit NCIS almost two years ago — but don’t look for it on the golf course.

“I left the show, I didn’t retire,” Harmon told PEOPLE in this week’s issue.

Instead, the father of two grown sons with wife, actress Pam Dawber, turned from 19 seasons as Special Agent Leroy Jethro Gibbs to a new passion project that has him digging through reams of World War II research.

Next month, the 72-year-old will publish his first book of historical fiction, Ghosts of Honolulu: A Japanese Spy, a Japanese-American Spy Hunter, and the Untold Story of Pearl Harbor. “I’ve always thought you can learn from history,” he tells PEOPLE. “Things tend to repeat themselves.”

The Ghosts of Honolulu reveals the true story of Douglas Wada, a Japanese-American born in Hawaii who became a spy for the US Navy, in his prime posing as a local newspaper reporter for what was then known as the Office of Naval Intelligence (ONI). Ultimately, Wada will hunt rival Japanese spy Takeo Yoshikawa across the Pacific after the coordinated attack on Pearl Harbor.

“This is the first story (leading) to the birth of what became (the real) NCIS,” says Harmon of the Naval Criminal Investigative Service. “These agents are truly a different breed. I hope there’s a story uncovered here that you don’t know. It was important work that they were doing, and no one knew about it.”

Harmon calls Wada a pioneer: “He was a very specific guy at a very specific time” with the right skills, working to uncover Japanese infiltration while trying to protect an innocent Japanese-American community targeted by his own government.

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Monty Brinton/CBS via Getty

The book is a labor of love between Harmon and co-writer Leon Carroll Jr., the latter of whom spent 20 years as a real-life NCIS special agent before becoming the CBS show’s longtime technical advisor.

While at the agency, Carroll Jr. “He was their No. 1 questioner,” says Harmon. “I was always asking him about the right way to do something or the way he would do something. There wasn’t a single trial I’ve ever been on this show that I didn’t talk to him about. So when that idea came up for me wanted it to be real. I said I’m not touching this without him.”

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Seth Poppel/Yearbook Library

Harmon’s television host father fought as a pilot in World War II, and Harmon had three years of ROTC training in high school as a youth before going on to play quarterback for UCLA. But he says being cast as Gibbs led to his current passion for naval espionage history.

“I was trying to research it, but there just wasn’t a lot of information about this agency,” he says. “It was actually asking questions and meeting people, and then it grew from that.”

Now, after almost two decades spent on NCIS, Harmon says he uses his free time to explore new careers. “I always find something to do,” he says. “I think for the longest time I was just tired, to be honest. My job was hard every week. I took it seriously, and there were a lot of people out there doing the same. I miss the camaraderie, the lunches around the table and hearing about people’s families and what they do outside of the show. But it’s a job.”

At this stage in his life, Harmon still feels lucky. “I have time to do whatever I want. I can plan dinner with the boys, we can go on a trip. I’m so happy, I never wake up without thinking that.”

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Ghosts of Honolulu: A Japanese Spy, a Japanese-American Spy Hunter, and the Untold Story of Pearl Harbor out November 14 wherever books are sold.

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Source: HIS Education