NCIS, the hit CBS show, is celebrating its 20th anniversary and is going strong with its 21st season. Over the years, the show has undergone many changes, with none of the original stars remaining as series regulars. Some shocking moments from the show include Tony nearly dying from the plague, Kate being killed, Ziva killing Ari, Gibbs avenging his family's murderer, Jenny's death, Michelle being revealed as the mole, Ducky having a heart attack, Ziva's death and secret daughter, Ziva's surprise return, Gibbs shooting McGee, and Gibbs leaving the show in season 19.