Mark Harmon’s absence from the NCIS Season 21 episode focused on Ducky was due to a combination of creative and personal decisions. Harmon, who played the iconic Leroy Jethro Gibbs, officially stepped back from the show in Season 19, leaving behind a lasting impact on the series. While his departure allowed for new storylines to emerge, his absence in such a pivotal episode, which heavily featured David McCallum’s Dr. Donald “Ducky” Mallard, was a conscious choice by the show’s producers. The episode aimed to highlight Ducky’s legacy and the evolving dynamics of the team without overshadowing the narrative with Gibbs’ return.

Another reason for Harmon’s absence was his desire to step away from the spotlight after years of playing a central role on NCIS. Reports indicated that Harmon sought more personal time and less involvement in the intense production schedule. The show’s producers respected his decision and chose to honor his character’s legacy while exploring new directions for the series, ensuring Gibbs’ influence remained part of the show’s heart, even if the character wasn’t physically present in the episode.