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

The former 'NCIS' star spoke to PEOPLE ahead of the release of 'Ghosts of Honolulu', his first historical non-fiction book

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

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

Instead, the father of two adult sons with his wife, actress Pam Dawber, pivoted from 19 seasons as Special Agent Leroy Jethro Gibbs to a new passion project that had him sifting through piles of research about World War II.

Next month, the 72-year-old will release his first historical nonfiction book, 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.”

Ghosts of Honolulu unearths the true story of Douglas Wada, a Japanese American born in Hawaii who became a spy for the U.S. Navy, in his greenest years posing as a local newspaper reporter for what was then known as the Office of Naval Intelligence (ONI). Ultimately, Wada would chase rival Japanese spy Takeo Yoshikawa across the Pacific following 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 really a different breed. I hope that there’s a story revealed here that you don’t know. It was important work they were doing, and no one knew about it.”

Harmon calls Wada a trailblazer: "He was a very specific guy in a very specific time" with a skill set to match, working to uncover Japanese infiltration while striving to protect the innocent Japanese American community being targeted by his own government.

Related: One of the Last Pearl Harbor Survivors, 101, Asks Next Generation to 'Keep a Record,' 'Be Positive'

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

While in the agency, Carroll Jr. "was their No. 1 interrogator," says Harmon. "I was always asking him about the right way to do something or the way he would do something. There wasn't any interrogation I ever was part of on this show that I didn't talk to him about. So when this idea came to me, I wanted it to be real. I said I wouldn't touch this without him." 

Related: A Call to Spy Highlights Virginia Hall, the Most Powerful Female Secret Agent in Modern History

Harmon's broadcaster father fought as a pilot in WWII, and in his youth Harmon had three years of ROTC training in high school before going on to play quarterback for UCLA. But he says it was his casting as Gibbs that led to his current passion for naval spy history.

“I was trying to research it, and there was just not much information about this agency," he says. "It was really asking questions and meeting people and then grew from that.”

Now, after spending almost two decades on NCIS, Harmon says he is using his free time to explore new pursuits. “I always find things to do," he says. "I think for the longest time I was just tired, to be honest. My workload was heavy every week. I took it seriously, and there were a lot of people there who did the same. I miss the camaraderie, the lunches at the tables and hearing about people’s families and what they’re doing away from the show. But it’s a job.”

At this stage in his life, Harmon continues to feel lucky. “I've got time to do whatever I want to. I can plan a dinner with the boys, we can take a trip. I'm so fortunate, I don't ever wake up not thinking that."

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

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