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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Cue a string of mind-bending flashbacks that ask the question: What if Gibbs had taken a different path? "There's almost an It's a Wonderful Life

quality to it," executive producer Gary Glasberg says. "Gibbs is given an opportunity in a millisecond to see what the world would have been like if key moments in NCIS history and lore had unfolded differently. We've taken some of those moments and we let people see them unfold differently."Indeed, Michael Weatherly, who plays Agent Tony DiNozzo, says the "brain-teaser, easter-egg-y... fabricated realities" will bring back characters who have long been dead. Among them: Gibbs' wife (Darby Stanchfield) and daughter, Gibbs' mentor Mike Franks (Muse Watson), and Ari (Rudolf Martin), one of Gibbs' enemies and the brother of fellow NCIS Agent Ziva David (Cote de Pablo).But the biggest surprise of all is connected to Gibbs' previously unseen mother. "I've got two words for you: Gibbs' mom," Weatherly says. "Two hundred episodes and now we get Gibbs' mom? I can't even begin to tell you the emotional weight that put on me. I was a little misty. More than misty — I was full-blown crying. You'd have to be a robot not to cry."

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But the episode isn't a total tearjerker. Weatherly says playing an alternate version of himself was quite fun. "I want the fans to relax and understand I didn't do anything strange to my hair," Weatherly jokes. "There's no fake sideburns like [in the episode] "Baltimore" last year. I didn't dye it black. Much to my chagrin, there's no Magnum P.I. mustache."But there are a few major differences [for Tony]," Weatherly continues. "There's a wedding ring and in some version of a time strand  — it's like a Star Trek episode for God's sakes — there are babies."Does that mean the fans who have long hoped Tony and Ziva would hook up will finally get to see their fantasy played out? Not exactly. Rather, in this alternate reality, Sasha Alexander's Kate Todd never died, and therefore Ziva and Tony's first meeting comes under much different circumstances. "There's a little glimpse into Ziva and Tony meeting now as opposed to then," Weatherly says. "I really enjoyed Tony in that scene. I wanted to make him the best version of him for that. That's a wonderful world that I see there."

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Viewers can get excited, however, about the possibilities for another fan-favorite couple, Abby (Pauley Perette) and McGee (Sean Murray). "I think people will be very satisfied," Glasberg says. "It touches on everyone in ways that you've always wanted to see."

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

Important enough for another 200 episodes? "We'll see what happens," Glasberg says. "As long as everybody is having a good time and as long as we can come up with fresh stories, we'll do it as long as we can."NCIS airs its 200th episode Tuesday at 8/7c on CBS.