Taking viewers behind the sealed doors of the military, NCIS follows the workings of investigators at the Naval Criminal Investigative Service. Led by the wise Special Agent Leroy Jethro Gibbs (Mark Harmon), a crack investigator with no qualms about bending the rules to find his answers, the team takes on cases involving terrorism, murder, espionage, and international arms-smuggling. Gibbs is aided by homicide detective Anthony Dinozzo (Michael Weatherly), forensic scientist Abby Sciuto (Pauley Perrette), medical examiner Dr. Donald "Ducky" Mallard (David McCallum), computer expert Timothy McGee (Sean Murray), and department head Jennifer Shepard (Lauren Holly). This collection presents the show's sixth season.