Mark Harmon and Muse Watson, NCIS Mark Harmon and Muse Watson, NCIS

What if?

That's the basic question at the heart of NCIS' 200th episode. The landmark hour (airing Tuesday at 8/7c on CBS) takes Leroy Jethro Gibbs (Mark Harmon) to his favorite diner for his usual morning cup of coffee. But he gets a whole lot more: Out of the blue, a hooded gunman approaches Gibbs, pulls his weapon and fires.

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But despite all the fun, Weatherly admits that an NCIS without Gibbs is a "daunting thought." "What would that be like if he were incapacitated to a point where he didn't behave the way we've come to expect him to behave?" Weatherly says. "It's an unthinkable world that we explore in this 200th episode, and I have to say, part of me was relieved when it was over. I was like, 'I just want to have a good cracking mystery next week!'"

Fear not, Gibbs isn't going anywhere. "Things get back to normal again," Glasberg says. "But I'd like to believe that as Gibbs enters the episodes that follow, he has a new perspective on why he's here, who he is and his importance to NCIS."

Look back at 200 episodes worth of photo from NCIS