Still not completely recovered from serious injuries incurred during the near-apocalyptic Season 3 finale, Leroy Jethro Gibbs (Mark Harmon) is at the outset of Season 4 retired from his position as head of the Naval Criminal Investigative Service, comprised of Ziva David (Cote de Pablo), Tony DiNozzo (Michael Weatherly), Abby Sciuto (Pauley Perrette), Tim McGee (Sean Murray) and Dr. Donald "Ducky" Mallard (David McCallum). Now the team is under the command of hotheaded DiNozzo...but not for long. Emerging from his self-imposed exile when his coworker Ziva is placed in jeopardy in the third season opener ("Shalom"), Gibbs assumes command of the team by the next episode ("Escape"), and within a few weeks is working at full throttle. However, despite several tantalizing revelations throughout the previous season, the intensely private Gibbs remains something of an enigma to the other NCI-ers--a fact that proves most frustrating to the unit's director, Jennifer Shepard (Lauren Holly). Among the clues about Gibbs' former life this season is the unexpected appearance of his sister Sarah, played by Troian Bellisario (daughter of series producer Donald P. Bellisario). Newcomers to the recurring-character roster include Scottie Thompson as Tony's girlfriend Jeanne Benoit, a DC-based doctor; and Susanna Thompson as Lt. Col. Hollis Mann, Gibbs' counterpart in the Army CID, with whom Gibbs implicitly begins a romance as the season winds down.