Warning - Though this is back story, it does contain some information that could be interpreted as spoilers. I tried to black out the stuff that isn't revealed until later seasons, but since Sailor Moon is a 'meta-series,' it would be impossible not to reveal anything.
Once upon a time, the Earth was a prosperous planet, protected by the Moon. The people of the Moon lived for a thousand years, so while they loved the Earth and it's people, they could only protect and observe from a distance.
At some point during this peace, Queen Serenity was blessed with a daughter, whom she named after herself. Like all living things in the universe, her daughter was born from a star seed, but Princess Serenity possessed a special star seed called a star crystal.
Star crystals are blessed with powers from their guardian planet; they give life to Sailor Senshi, who are destined to protect life in the galaxy. Each is unique; unknown to Serenity, her crystal is unmatched in power and grants the power of rebirth.
The Princess Serenity grew up watching Earth from afar, and one day, she saw one of Earth's princes, Endymion (Darien in the crappy dub). Though it was forbidden, the two would meet in secret and fell deep in love.
During one such rendez-vous, the queen of Earth spotted the lovers. Queen Beryl loved Endymion as well, and in a fit of jealousy, the queen fell let the darkness of Chaos take over. Her corrupt heart wished only to punish the princess.
The Queen manipulated the people of Earth. She used their jealousy of the Moon's inhabitant's long life spans to instigate a war against the Moon Kingdom. Even Endymion's own army and generals turned against him. The battle was brutal. While Sailor Mars, Mercury, Venus and Jupiter fought to preserve as many lives as possible, Queen Beryl demanded the people of Earth fight to the death. Forbidden to leave their posts, Sailor Pluto, Uranus, and Neptune watch in horror from a distance. They know that Sailor Saturn, the Soldier of Destruction, will be reborn to end both kingdoms.
Sailor Saturn dropped the Glaive of Silence and both Kingdoms were destroyed. The Sailor Senshi lost their lives. Endymion, still unwilling to fight against the Moon, joined Serenity. Beryl was infuriated. She directed her rage at the princess, attacking with full force. Endymion used his body to shield Serenity.
Endymion bled to death in Serenity's arms. In her grief, not only for her dead lover but also for the downfall of both kingdoms, Serenity took her life using a holy sword.
Her mother, heartbroken but hopeful, used Princess Serenity's star crystal, the Silver Crystal, to send the dead to the future. Using the crystal took the last of the Queen's power, and she died. Having no star crystal of her own, the queen could not be reborn.
Princess Serenity, Endymion, and the Sailor Senshi who died fighting were sent to modern day Japan, a time of peace. Named Usagi (Bunny) by her Earth parents, the princess retained no memories of her past life and grew up an average middle school girl.
Until the darkness appears on Earth again... And that is where the story begins.
