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On NCIS, no case ever stumps naval investigator Leroy "Jethro" Gibbs, but in real life, Mark Harmon, 66, has long been befuddled by one mystery: fame. "My parents kept things real — I had no idea they were famous," says the son of sportscaster Tom Harmon and actress Elyse Knox. "In fact, it didn’t hit me until one day when I was riding in the car with my father in Ann Arbor, MI — I was maybe eight and could barely see above the dashboard — and we stopped at a crosswalk. Suddenly, we were surrounded by people who recognized my dad and were really thrilled to see him. I remember looking at this man I thought I knew so well and thinking, Who are you?"

Millions of people ask the same question about Mark to this day. He’s the star of TV’s No. 1 drama yet he maintains an intense sense of privacy. He’s raised Sean, 30, and Ty, 25 — his sons with wife Pam Dawber, 66, with whom he recently renewed his vows in a quiet backyard ceremony — to keep similarly low profiles. "It’s not even a choice. It’s who we are," Mark explains. "We stay home. A lot. I’m not a Twitter guy or a Facebook guy. Our sons aren’t into that, either."

Interviews with Mark as well as his family members, friends, and co-workers reveal a portrait of a consummate professional who values his home life above all else — and keeps it separate from his career, even during periods of severe personal and professional stress. "Mark is a shining example of the way it should be done," NCIS co-star Sean Murray exclusively tells Closer. "He’s incredibly grounded, works hard, and knows that family counts most."

Those are lessons Mark learned early in life. "My father wanted me to be self-motivated," he recalls. "I was taught the only way to go about getting something was with everything you had." That he did, becoming a football star for UCLA and working as a roofer, carpenter, and shoe salesman before getting into acting at the urging of Ozzie Nelson, his sister Kristin’s father-in-law (she’d wed Ozzie and Harriet’s son, Ricky, in 1963).

"I’d never been on a set before," Mark says of his 1973 acting debut on the Nelson family’s sitcom Ozzie’s Girls. "My character comes to the door in a gorilla outfit, and I got to say, 'My name’s Harry King Kong. Which way to the Empire State Building?'"

It was an inauspicious start, and the situation didn’t improve as Mark toiled in short-lived shows like the rescue drama 240-Robert (he played Deputy Dwayne "Thib" Thibideau) and the overheated soap Flamingo Road. He romanced Road co-star Cristina Raines for four years, but they split in 1984. "She will always be a very special lady to me," he said.

But Mark was looking for true love. "When I marry, I want it to be everything this town says it can’t be," he boldly declared in 1986. "I’m not into catting around." He met his match in Pam, who was introduced to him at a party thrown by a mutual friend. She had made a list of qualities she was seeking in a man — strength, intelligence, independence, sensitivity, sensuousness, warmth, and a sense of humor — and Mark ticked off every box. They wed in 1987, and Sean and Ty came along soon afterward.

While Pam had enjoyed success on the small screen with the sitcoms Mork & Mindy and My Sister Sam, she put her career aside to raise the kids. "I had children and it was like, 'I’m not going to chase this fame thing,'" Pam says. "I wanted to drive my kids to school. I wanted to be there for their birthdays and bring cupcakes and doughnuts. And I was very happy to do it."

Meanwhile, Mark’s career was really starting to cook with his breakout role as a heartthrob plastic surgeon on the critically worshipped hospital drama St. Elsewhere. But after three seasons, he felt lost in the teeming ensemble. "Mark’s a leading man, so I could understand his frustration," producer Tom Fontana tells Closer. "After all, he was a quarterback." In a groundbreaking plot twist, his heterosexual character contracted AIDS and left the show, but there were no hard feelings. "He did a great job with a challenging storyline," says Fontana. "We’ve stayed friends."

Trouble brewed on the home front, however: Kristin had gotten mixed up with drugs during her marriage to Ricky, and after the rocker died in a 1985 plane crash, she went off the deep end. Mark engaged in legal wrangling to win custody of her son, Sam, then 12, and convinced Kristin to enter rehab. "Nobody but my brother could have talked me into this, because I trusted him," Kristin said. Ultimately, Mark dropped his bid for custody and was granted visitation and Sam remained close to his mom until her death in April at age 72 from a sudden heart attack.

Mark and Pam have steered Sean and Ty clear of drugs and other temptations by being hands-on parents. That was another bit of hard-won wisdom for Mark. "I was in the jungles of New Guinea making a not-very-good movie [Till There Was You] when my firstborn took his first steps," he says. "No job is worth missing life’s most important moments."

He’s since served as a sterling role model to his sons, proving himself a real-life hero by saving two teens trapped in a wrecked Jeep near his LA home in 1996. Mark smashed the windshield with a sledgehammer and extracted the youths before their vehicle exploded. "I owe everything to him," said one of the survivors. "I have no doubt I wouldn’t be alive."

Mark’s career hit a lull in the 90s, but a guest shot on The West Wing put him back on the TV map, and in 2003 along came NCIS. The show had a rocky launch, as Mark clashed with creator Donald Bellisario over scripts arriving late and a zoo-like atmosphere on the set. Bellisario left and was replaced by Gary Glasberg, who became one of Mark’s closest friends and ran a tight operation, but Glasberg tragically died in his sleep at 50 in 2016. "That hit Mark very, very hard," an insider tells Closer.

Still, Mark righted the ship and continues to weather on-set storms, like co-star Pauley Perrette’s controversial recent departure amid allegations that Mark’s dog bit a crew member. "Mark’s very sensitive to the people around him," co-star Rocky Carroll assures Closer. "The reason it’s such a great working atmosphere is he wouldn’t have it any other way. You can just tell the kind of upbringing he had."

Mark leads by quiet example, à la Gibbs. "Like his character, he’s a functional mute," jokes Michael Weatherly, who left NCIS to star in his own CBS hit, Bull, in 2016. "Mark is a very old-fashioned, old-school gentleman, and I’ve learned a great deal at his side."

When fans recognize Mark — like they did his dad all those years ago in Ann Arbor — he remains the ultimate gent. "It’s hard to walk through airports even in the most out-of-the-way places without being surrounded by people who love the show — and that’s nothing to complain about," he says. "I’ve done TV shows where I walk through airports apologizing." No doubt he handled that with equal grace.