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.