NCIS is a crime procedural television show based on the real-life Naval Criminal Investigative Service that investigates felony crimes involving the U.S. Navy and Marines.

NCIS began as a spin-off of another show that also followed an arm of the Navy judicial system, “JAG.” It did not take long for the new show to gain its own following, and has over six seasons under its belt. It is also being shown in syndication reruns.

The show follows a team of investigators given the authority to operate outside of the normal chain of command. Leading the team is Special Agent Jethro Gibbs, played by veteran TV actor Mark Harmon. Other members include former homicide detective, Special Agent Tony DiNozzo and Ziva David, a liaison officer from Mossad, the Israeli intelligence unit. Assisting are medical examiner Dr. Donald “Ducky” Mallard and Goth-oriented forensic specialist Abby Sciuto.

Cases have included terrorist and bomb threats, the death of a presidential aide and the death of a celebrity while filming a show on a Marine Corps base. The cast and crew attribute the success of NCIS to its character-driven plots, ensemble cast and comedy.

NCIS has gained viewership since it premiered in 2003. When it began, it had an average viewership of nearly 12 million. Following its sixth season, its average viewership had grown to nearly 18 million, and it often is a strong second to American Idol.