The Silver Millennium
Season 1 revealed history · The Moon Kingdom before the present-day story
The Silver Millennium is the lost Moon Kingdom remembered during the first season. Queen Serenity ruled there, Princess Serenity loved Earth's Prince Endymion, and the Sailor Guardians protected the Princess.
For most of the season, nobody in present-day Tokyo remembers that life clearly. Luna is searching for the Princess, Mamoru is following fragments of memory through dreams, and Usagi thinks she is simply a newly recruited superhero.
Why it matters
The Silver Millennium is not just ancient backstory. Its return reorganizes the relationships the show has already spent months building in the present.
The Cataclysm
SPOILER!
Season 1 revealed history · Beryl and Metalia destroy the old world
The Dark Kingdom's attack destroys the Moon Kingdom and separates Serenity from Endymion. Queen Serenity uses the Silver Crystal to seal the threat and send the people she loves toward another life on Earth.
The present-day cast therefore inherits a war they do not initially remember choosing.
What changed
Once those memories return, Queen Beryl is no longer just the boss behind a string of energy-stealing monsters. She is part of the reason Usagi, Mamoru and the Guardians had to begin again at all.