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.

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.

Gibbs gets a visitor at Camp Pendleton from DC Headquarters in a new NCIS: Origins Season 2, Episode 3 sneak peek. See the very special reunion here.

NCIS: Origins Season 2, Episode 2, "Who by Fire," flips the script on what audiences know about Mark Harmon's Leroy Jethro Gibbs character.