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Along with the late David McCallum, Mark Harmon is arguably the actor who's most commonly associated with the "NCIS" franchise. His character, Leroy Jethro Gibbs, debuted on "Jag" before gracing 19 seasons of "NCIS." As of this writing, Gibbs is also the only character to receive his own prequel series, which we'll cover in more detail later. In short, though, Harmon is a poster child for this franchise, and that's why fans were shocked when he parted ways with the hit procedural in 2021.

Harmon was in his 70s when he left "NCIS," but he didn't quit because he wanted to retire and enjoy the fruits of his labor. In fact, the Gibbs star stayed on board as an executive producer and kept busy working on some passion projects. He then returned to the "NCIS" universe in a more substantial capacity, even...