The 400th episode of NCIS will delve into the backstory of Leroy Gibbs, revealing his psychological research and the first meeting with his friend Ducky. Actor Sean Harmon, son of protagonist Mark Harmon, will reprise his role as the young Gibbs in this special episode. Mark Harmon expressed his excitement about the episode, stating that it was a reward for long-time fans of the show. The episode also features a glimpse of young Gibbs meeting medical examiner Ducky Mallard. Sean Harmon has previously portrayed young Gibbs multiple times, while Adam Campbell played young Ducky.