“NCIS” has been on air since 2003, chronicling over 20 seasons of action and adventure for the franchise’s special agents. While the cast has rotated quite a bit over that time, Leroy Jethro Gibbs, played by Mark Harmon, has been a staple of the long-running drama. He’s one of the original characters introduced on the show, officially exiting after his retirement from the force in Season 19. As the main supervisor for the other agents, Gibbs keeps the others in line while helping everyone navigate difficult cases and situations. He’s more reserved than other members of the team, choosing to use few words to get his intentions across and watching those around him rather than inserting himself into unnecessary conversations.

During Gibbs’s time on the series, we learn more about his personal life, including what drove him to become a special agent, and now, fans who love the character can go back in time with “NCIS: Origins” to see where his story begins. The spin-off (which does slightly retcon some of Gibbs’ backstory) takes place shortly after the murders of Gibbs’s wife and child, right when he’s beginning his career with the NCIS, then called the Naval Investigative Service.

While Harmon’s son, Sean, plays the younger version of Gibbs in the main series across seven episodes, he didn’t take up the helm in the prequel series. Instead, Austin Stowell, who may look familiar to you from his other projects, is the new young Gibbs. If you’re among those curious why Sean didn’t return, this is why Mark Harmon’s son doesn’t play young Gibbs on “NCIS: Origins.”

While Sean Harmon, Mark Harmon’s son, portrayed a younger version of his father’s “NCIS” character seven times in the main series, he did not return to play the character in “NCIS: Origins.” However, that doesn’t mean he’s not as involved with the spin-off show just as much as Mark is.

In an interview with Variety, Mark Harmon detailed why his son didn’t reprise his previous appearances to play the lead role in the spin-off. “You have to come to decisions about what you want to do in life, or what you think’s important. I just think he made a choice and had to make a decision in some ways,” the actor said. “He likes executive producing.” In addition to his own role as an executive producer on “NCIS: Origins,” the elder Harmon provides voiceover narration as Gibbs throughout the series.

“NCIS: Origins” isn’t the first time Sean Harmon has worked behind the camera. In addition to performing as a stunt double, he’s also been a stunt coordinator on shows like “Golden Revenge” and “Breakwater,” and films like “Alone” and “Dumb and Dumber To.” He’s also had a different role in the “NCIS” universe, portraying Charlie Connor in an episode of “NCIS: Los Angeles” in 2015.