The NCIS spin-off still lacks a title.

But the series, which will premiere on its own this fall, gets jump-started tonight, as NCIS kicks off a two-part episode, "Legend," that introduces fans to the two main characters in its spin-off.

Played by Chris O'Donnell and LL Cool J, respectively, they are agents G. Callen and Sam Hanna.

Series creator Shane Brennan recently spoke on each characters. He described them to Entertainment Weekly:

On Callen: "[He] has worked for the CIA, the FBI, DEA, and everyone recognizes the same thing: he was born to do undercover work. He's brilliant, he's amazing, and they all fired him because he breaks the rules. He found his home at NCIS because Macy understands that he's a loose cannon.

On Hanna (pictured): a former Navy SEAL that has ''seen service in every incident, skirmish, battle that the U.S. has been involved in in the past 20 years. He's highly experienced, he's been wounded... perhaps not quite as quick as he used to be, but he's much wiser."

Matt Richenthal is the Editor in Chief of TV Fanatic. Follow him on Twitter and on Google+.