In the hour, as the team investigates an attack on an American couple in Iraq, Gibbs (Mark Harmon) heads to Walter Reed Medical Center to connect with a wounded Special Ops sniper (guest star Taye Diggs), who is the sole survivor of an ambush in the same area six months prior, and who is now being treated for post-traumatic stress syndrome.
"The 300th is a really special episode for us," NCIS executive producer Gary Glasberg tells TVGuide.com. "We ended up building this very patriotic, very emotional story line. ... It's certainly going to tug at people's heartstrings, that's for sure."
As part of the Walter Reed plotline, NCIS producers teamed up with MusiCorps, a nonprofit rehabilitation program that uses music therapy to treat wounded warriors - meaning, yep, Diggs' vocal chops will be on display as well as his acting. Which should be no problem for the Broadway veteran.
"Because there is a musical element to it, I needed someone with his talent," Glasberg says. "And he liked what he read and was eager to come and help us out. The moments between him and Gibbs are really special."
Diggs' character connects with Gibbs "on an emotional level and a psychological level," according to Glasberg, as Gibbs - who has spent much of this season coming to terms with his own near-death experience - realizes he and the wounded veteran have covered some common ground.
"The two of them sort of learn from each other," Glasberg says. "It absolutely connects to the things [Gibbs has] been thinking about and struggling with all season long, and the things that he's been working with [Jon Cryer's] character on. ... It definitely connects to all of that and of course even goes further back into Gibbs' experience as a sniper, and into some of our folklore."
The episode also features the return of Dr. Grace Confalone (Laura San Giacomo), Dr. Taft's (Cryer) confidante who accompanies Gibbs to Walter Reed.
"It's got a little bit of everything for our fans," Glasberg says of the tercentenary episode. "There's certainly some action, and then this very heartwarming, patriotic story as well. We're all very proud of it."
Check out a sneak peek of NCIS' 300th episode here: