NCIS has somehow been on the air for over two decades now, and it just keeps expanding. What started as a show about a Navy investigative team has turned into a whole universe of spin-offs. Some stuck around for over 10 years, while others barely made it past Season 1. And here’s the part that trips newer audiences up: NCIS wasn’t even an original idea. The series actually branched out of JAG, a ’90s military legal drama that casual fans of the current lineup have probably never watched a single episode of.

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