The main cast remained consistent, with the notable addition of as a series regular. Portrayed by Leroy Jethro Gibbs Mark Harmon Lead Special Agent Anthony "Tony" DiNozzo Michael Weatherly Senior Special Agent Ziva David Cote de Pablo Mossad Liaison Officer Timothy McGee Sean Murray Junior Special Agent Abby Sciuto Pauley Perrette Forensic Scientist Dr. Donald "Ducky" Mallard David McCallum Medical Examiner Leon Vance Rocky Carroll NCIS Director Notable Guest Stars: Ralph Waite as Jackson Gibbs (Gibbs' father). Michael Nouri as Eli David (Ziva's father/Mossad Director).

The latter half of the season focuses on Ziva David’s torn loyalties between NCIS and Mossad. This culminates in a violent confrontation between Tony DiNozzo and Ziva's boyfriend, Mossad agent Michael Rivkin.

The sixth season of premiered on September 23, 2008, on CBS and ran for 25 episodes. It is noted for significant shifts in team dynamics, deep dives into character backstories, and the introduction of the NCIS: Los Angeles spin-off. Major Plot Arcs