Harmon will return in the flesh for a special crossover event on Tuesday, Nov. 11. For one night only, “NCIS” and “NCIS: Origins” are swapping time slots; the spinoff will air first at 8 p.m. ET, followed by the original series.

NCIS: Origins revealed more of the backstory of Special Agent Leroy Gibbs ( Austin Stowell) and NIS medical examiner Dr. Donald “Ducky” Mallard ( Adam Campbell) on tonight’s episode, and it was a surprise how much Gibbs’ friendship meant to Ducky and how little of it Gibbs remembered.

Mark Harmon, who spent 18 seasons on "NCIS," is reprising his role as special agent Leroy Jethro Gibbs for a crossover event.

But arguably the biggest reveal came with learning why Ducky talks to bodies, which we saw the late David McCallum did when he was playing him for so many years on NCIS. Origins co-showrunner Gina Lucita Monreal and Campbell opened up to CinemaBlend about telling this “difficult” story and more.

Origins would deliver its own tribute to fan-favorite Donald “Ducky” Mallard, it was hard to know what to expect from this venture or whether Adam Campbell could reprise the role in a way that aligned with this spinoff.

As NCIS: Origins moves forward, fans can expect emotional complexity, professional stakes, and romance that’s both teased and restrained. Gibbs’ journey isn’t just about love — it’s about the making of a leader, a partner, and an agent whose choices reverberate across the franchise.

NCIS: Origins” paid tribute to the beloved Donald “Ducky” Mallard in its latest episode, bringing back actor Adam Campbell to the franchise

Adam Campbell's "NCIS" character, who has appeared on the mothership in flashbacks as the young version of David McCallum's Ducky, is sent to work with our "Origins" team to see if they need an in-house medical examiner. But the crew at Pendleton is skeptical of his true motives.

"NCIS: Origins" honors David McCallum in a tribute episode Oct. 28 featuring young "Ducky" Mallard's return. USA TODAY gets an exclusive sneak peek.