Taking place in the feudal era of Japan, there are 2 clans locked in a brutal war, the Byakuro clan and the Kabusu clan. The Kubusu clan staged an assault to try to kidnap the Byakuro clan’s infant princess but failed. A ninja who is loyal to the Byakuro clan saved the princess and disappeared with her to raise her as a ninja. Several years went by and there was a new crop of trainees. As part of their training, they had to infiltrate Byaruko castle which is now occupied by the Kabusu to retrieve a sacred scroll. When met by Kabusu warriors, they learn that one of their fellow trainees is the princess and their mission changes just retrieving the scroll to also protecting the princess.
Ninja Cadets is only a 2 episode OVA so it’s not a major timesink if you decide to watch it. It’s a show from 1996 which does reveal it’s age but it’s still a decent show nonetheless. Ninja Cadets actually has the honor of being the first anime DVD released. Probably not what you were expecting for the first ever but it indeed was the first.