“NCIS: Origins” may not stray far from the DNA of its flagship, but the updates to the formula make revisiting Leroy Jethro Gibbs’ past a “darker” chapter for the beloved CBS franchise.

The new series turns the clock back to 1991, when Gibbs first joined NCIS as a special agent at Camp Pendleton. The two-hour premiere follows as the stoic agent steps into his own with a new team, while mourning the loss of his wife and daughter. Harmon narrates every episode, dropping insight into the mysterious character’s mindset for the first time — and dropping hints as to what may be coming next.

For co-showrunners David J. North and Gina Lucita Monreal, the Gibbs-centered prequel series offered the right opportunity to tackle a more “character-centric” version of the crime procedural.

“The show we’re making now has the heart of the mothership but a little darker,” North told TheWrap. “The...