https://www.etonline.com/ncis-mark-harmon-says-episode-400-is-a-reward-for-the-fans-exclusive-156808

The "Everything Starts Somewhere" episode flashes back to Gibbs' introduction to NCIS and his first meeting with David McCallum's Ducky. Harmon's son, Sean Harmon, will reprise his years-long role as Young Leroy Jethro Gibbs.