This is a complicated timeline of the events in the manga Sailor Moon.
In the distant past, during the Silver Millennium, Princess Serenity of the Moon Kingdom fell in love with Prince Endymion of the Earth Kingdom. When both kingdoms were attacked, Prince Endymion rushed to save Princess Serenity's life, sacrificing himself in the process. Princess Serenity committed suicide out of grief. Her mother, Queen Serenity, would give her life to allow both lovers to reincarnate as Earth people some two thousand years later.
In modern-day Tokyo, Usagi Tsukino is a middle school student. One day, she meets a mysterious cat named Luna, who reveals to Usagi that she has incredible mystical powers triggered by the phrase "Moon Prism Power!" With her new-found powers, Usagi vows to protect Earth as the heroine Sailor Moon.