The protagonist of Sailor Moon, Usagi Tsukino (Serena in English adaption), an ordinary, ditzy, 14-year-old girl — or so she thinks — discovers a talking cat named Luna, who reveals Usagi's identity as "Sailor Moon", a special warrior with the destiny of saving the planet Earth, and later the entire galaxy. Usagi must now find the Moon Princess and protect Earth from a series of antagonists, beginning with the Dark Kingdom that had appeared once before, long ago, and destroyed the Kingdom of the Moon.
When the dark nemesis attacked the kingdom, the Queen sent the Moon Princess, her guardians, advisors, and her true love into the future to be reborn. As Usagi and Luna battle evil and search for the Moon Princess, they meet and awaken the other Sailor Senshi, incarnations of the Moon Princess' protectors, and the mysterious Tuxedo Mask.
As the series progresses, Usagi and her friends learn more and more about the enemies they face and the evil force that directs them. The characters' pasts are mysterious and hidden even to them, and much of the early series is devoted to discovering their true identities and pasts. Luna, who teaches and guides the Sailor Senshi, does not know everything about their histories either, and the Senshi eventually learn that Usagi is the real Moon Princess. The Moon Princess' mother had her reborn as a Sailor Senshi to protect her. Gradually Usagi discovers the truth about her own past life, her destined true love, and the possibilities for the future of the Solar System. (source: Wikipedia)