About Ninja Gaiden

忍者龍剣伝, Legend of the Dragon Sword

Gaiden can mean side-story or letter. For most games, Gaiden is used to denote that the game is a side-story in a series of games; not necessarily cannon or even part of established timeline. However, in the case of this game's title is actually means Letter. The story kicks off when the protagonists finds a letter written by his father who was recently killed in a duel.