Jethro Gibbs cares deeply about Mark Harmon.

After playing the special agent on “NCIS” for 18 years, Harmon left the series as a series regular in 2021, but remained a series regular. However, he was never far away – it was too important for him to keep it completely in his rearview mirror. When his son brought him the idea for a prequel, the two even approached showrunners David J. North and Gina Lucita Monreal directly to get the idea off the ground.

“Is there a story to tell? Definitely. Will it be different than what people know? Definitely. “I couldn’t be more excited about Gina and David, who they represent and their history with his show,” he says of “NCIS: Origins,” the prequel set to debut Oct. 14. “They were the best I've ever worked with on this show, and I love what they've done and what they've worked very hard on and tried to push forward. My job is to try to protect that.”

It hasn't been an easy journey, says Harmon, but he's happy to now enjoy the peace and quiet before the series premiere.

“It will be hard work and you know there will be changes. You know there are going to be things you can control and things you have to fight for and things you can't control,” he says. “Right now it feels like we’ve been fighting for everything, but it feels good.”

The series begins in 1991 at Camp Pendleton, shortly after the murder of Gibbs' wife and child. Austin Stowell will portray young Gibbs, a role once played by Harmon's son Sean Harmon in seven episodes of “NCIS.” Sean serves as executive producer for the prequel.

“He’s an actor, writer, producer and surfer, he does a lot of things. You have to make decisions about what you want to do in life or what you think is important. I just think he made a choice and in some ways had to make a decision,” Harmon says of why his son didn't return to the role. “He likes to executive produce.”

As for Mark Harmon, he's always available on set to answer any cast questions – and he knows he will be, having played the role for so long. Because he completely trusts the show and the team behind it, he didn't hesitate to re-enter this world.

“Gina and David had asked me to do the voice-over and that was always part of my understanding. “I don’t know if it was initially clear how this would work,” he says diversity. “Then the question was: How do you do it? Is he standing there with his finger on his chin? Is he writing a book?”

Harmon will appear in the first episode, but all details about it are being kept secret. “This was supposed to be a surprise,” Harmon jokes about the cameo. North shared the news during a panel discussion last month.

Whatever the case, Harmon just wants everyone to continue to love life on set – which has been the case so far.

“There has to be changes. That's life, right? Everyone works hard and everyone has a great attitude, and then the whole thing is broadcast and judged. This ruling will change some things,” he says. “But our job is to do the work and really try to constantly keep the bar at a certain level as we create the next episode.”

“NCIS: Origins” premieres Monday, October 14, at 9 p.m. ET on CBS.