The episodes "Ice Queen" and "Meltdown" first introduced Mark Harmon's - who almost replaced Don Johnson on Miami Vice - Gibbs and his dedicated team of agents.
This includes David McCallum as Dr. Donald Mallard, AKA "Ducky." McCallum was already a TV icon long before NCIS, having appeared in the '60s spy series The Man from U.N.C.L.E.
While the star hasn't stepped away from the series, his appearances have been limited since NCIS season 16.