As CinemaBlend notes, "NCIS" actually started as a spin-off for "JAG" – another legal drama, short for Judge Advocate General, that aired on CBS for 10 seasons from 1995-2005, according to IMDb.
"NCIS" was introduced through a pair of backdoor pilot episodes of "JAG" called "Ice Queen" and "Meltdown," per TV Guide. In these Season 8 episodes, viewers were introduced to what would become the core cast of "NCIS" as they not only work to solve the murder of a lieutenant but also try to thwart a terrorist attack.
Both "JAG" and "NCIS" were created by Donald P. Bellisario, a famous producer and screenwriter, for several investigative television shows, and while they may seem like similar shows with similar premises on the surface, "NCIS" has a distinctive streak of comedy that sets it apart from the series it spun off from. Additionally, there are some cast members from "JAG" who have appeared on "NCIS" as well.
