Sailor Moon is the title of a Japanese media franchise created by Naoko Takeuchi.
The story revolves around the reincarnated defenders of a kingdom that once spanned the solar system, and around the evil forces that they battle.
The major characters—called Sailor Senshi (literally "Sailor Soldiers")—are teenage girls who can transform into heroines named for the moon and planets.
The elements of fantasy in the series are heavily symbolic and often based on mythology.
The Sailor Moon manga was preceded by 'Codename: Sailor V', which centered around just one Sailor Senshi. Takeuchi devised the idea when she wanted to create a cute series about girls in outer space. When Sailor V was proposed for adaptation into an anime, the concept was modified so that Sailor V herself became only one member of a team. The resulting manga series was a fusion of the popular magical girl and sentai genres of which Takeuchi was a fan, making Sailor Moon one of the first series ever to combine the two.
The manga resulted in spinoffs into other types of media, including a highly popular anime, as well as musical theatre productions, video games, and a live-action (tokusatsu) series. Although most concepts in the many versions overlap, notable differences occur, which results in only limited continuity between the different formats of the story.
For full info on the storyline, characters and background of the Sailor Moon series I recommend you visit the Wikipedia page on Sailor Moon:
http://en.wikipedia.org/wiki/Sailor_moon
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