Mark Harmon has co-written a non-fiction book detailing the world of NCIS’ predecessor decades prior to the CBS police procedural.

NCIS veteran Mark Harmon is behind an upcoming non-fiction book about the Second World War operation led by the ONI (Office of Naval Intelligence), the predecessor of the Naval Criminal Investigative Service.

The 71-year-old has teamed up with tech advisor and former Special Agent Leon Carroll Jr on the book that is to give plenty of insight into how NCIS was formed.

Harmon has paid tribute to the CBS drama, praising NCIS for shining a light on the historic organisation.

Harmon said in a statement: “I feel compelled to take part in opening up the history and real story of what became NCIS.

“When I first started this show, there was not much information to be found by research.

“NCIS agents are public servants at the highest level and many have come and gone through this life with no one knowing anything about who they are or what they do. And now that story gets told. All because of a TV show.”

Harmon and Carroll Jr’s narrative non-fiction, after studying “long-buried historical documents”, tells the story of two men.

The first is Douglas Wada, the only Japanese American agent in naval intelligence, and the second is Takeo Yoshikawa, a Japanese spy who was sent to Pearl Harbor to get information on the US fleet.

Wada posed as an undercover newspaper reporter, translating wiretaps on the Japanese Consulate and interrogated America’s first captured POW of the Second World War.

On the other hand, Yoshikawa worked with the consulate as a junior diplomat and collected information to send to the Japanese Navy’s Admiral Yamamoto.

The book, which will be entitled Ghosts of Honolulu, depicts “the world-changing cat-and-mouse games played between Japanese and U.S. military intelligence agents.”

Speaking about their project, Carroll Jr added: “I hope this book will give readers a different look at NCIS as an agency and that we are much more than the homicide-of-the-week.

“Their name NCIS was chartered in 1992, 90 years after the establishment of ONI to emphasize the criminal mission of the agency…

“This book is intended to be the first in a series that will give an inside look into the inner workings of accomplishing that mission.”

Harmon made his exit as Agent Leroy Gibbs in the middle of season 19 and sadly hasn’t been seen since as he went to start a new life in Alaska.

Despite his exit, Harmon remains an executive producer of NCIS which will return for season 21 later this year.

Ghosts of Honolulu: A Japanese Spy, A Japanese American Spy Hunter, and the Untold Story of Pearl Harbor will be released on November 14 on Amazon Prime Video.