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.

Gibbs (Austin Stowell) and Ducky Mallard (Adam Campbell) already have a personal history together, which is why Ducky's called in as a liaison between Gibbs' people at Camp Pendleton and the D.C. NCIS office — and also why Ducky gifts Gibbs upon their parting with a special object that they both admired while out on the town together: a boat in a glass bottle.

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.