Honestly, the past episodes of Sailor Moon have been extremely tied together. Since the appearance of Makoto and the naming of Usagi as the Sailor Soldier's leader, it feels that the acts are meant to be seen one after the other.
The episode opens right after Sailor Venus (Act 8) ended, with Mamoru Chiba or Tuxedo Kamen receiving the full force of the attack that was for Sailor Moon.
He was hurt badly, and while he is unconscious he has the vision of a woman, he finally realizes that Sailor Moon is the same woman of his dreams; she is Serenity. And he remembers more, his name was Endymion.
When Sailor Moon listens this name, she starts to remember something, but it is only when Mamoru completely looses consciousness that she explodes in grieving, giving in to her emotions, giving into her power. The new (and horrible) tiara she had breaks revealing the symbol of the moon Kingdom on her forehead.
Then she transforms into Princess Serenity, the real Princess. Usagi is the Princess the girls were looking for, and now that her identity was exposed, Luna remembers everything as well. She was meant to protect the Princess. She was always there to protect Usagi.
Serenity, now remembering that she has always loved Mamoru cries over his body, and one of her tears transforms into a crystal. Mercury analyzes this and confirms that it is the Silver Crystal they have been looking for. The Princess keeps crying over Mamoru's body, and it seems that one small piece of the crystal falls into his body.
That is the moment when Kunzite decides to attack the pair, but the soldiers are there to protect the princess. They attack him and this time their attacks seem to affect him a little. But Kunzite is rescued by Queen Beryl, she overpowers the soldiers and steals Mamoru's body from Serenity's arms.
Queen Beryl takes him back with them, and Sailor Moon, Serenity, is restrained by the girls while she screams his beloved name: Tuxedo Mask, Mamoru, Mamo-chan. Her paramour is gone.
Luna explains: Once upon a time when the earth was just one continent, and there was a kingdom on the Moon begun this story. The Moon kingdom was a beautiful realm protected by the power of the Silver Crystal, and the heir of the Moon Kingdom was Princess Serenity. She was supposed to stay always on the moon, but she loved watching the earth from afar, she loved watching its beautiful green pastures and incredible blue skies. But what she loved more was watching the earth's Prince. They were never supposed to meet; they were never supposed to talk, and above all they were never supposed to fall in love.
Tragedy marked Their love. The Kingdom of Earth misguided by the forces of evil, began a revolution against the Moon Kingdom, and while trying to protect the Princess, Endymion, Prince of Earth died. This story is all that they remember, and it is what Serenity or Usagi dreams of constantly.
The girls appear at the Tsukino residence after picking up Luna from the arcade. The poor cat is lamenting not trusting Usagi's instincts and not believing in Tuxedo Mask, she thinks they could have saved him. The girls tell her that they will rescue him but that first they have to cheer Usagi up.
At the Tsukino residence, Mama Ikuko tells the girls that Usagi hasn't left her room. And when they knock on Usagi's door they meet with a surprise.
Usagi has uncommonly long hair. Venus and the girls cut it and then she explains that it's probably because she had incredibly long hair while living on the Moon and that her body is probably reacting to her being the reincarnated Princess.
Usagi says the does not want to become Serenity; she is Usagi! And Rei tells her that she was reborn as Usagi for a reason. She is and will always be Usagi first.
Luna then says that they have to journey to the Moon. The only way for them to discover more about the enemy and what can they do to save Tuxedo Mask is to visit the ruins of the Moon Kingdom. In the next episode, the girls are going to the Moon!