The original Anime had this same episode, based on the manga where there is police barricade because of a Princess with her mysterious box the enemy thinks is the Mysterious Silver Crystal. Usagi wants to go and disguises as a princess and dances with Tuxedo Mask. The Princess who looks like Umeko with nerdy glasses gets possessed by Nephrite. Usagi and Tuxedo Mask fall from a balcony and sail down due to a Parasol. Moon, Mars and Mercury exorcise Nephrite's copy from the Princess and we find out that she didn't have the Silver Crystal. In the end, Mamoru kisses Usagi while sleeping and she says it reminds her of something.
So this episode pretty much follows that, except of just seeing Nephrite, the Sailors meet all four Shittenou. I was never fond of Usagi's first kiss as Mamoru kissed her when she was drunk in the original Anime. In the dub, they made it into a dream. In the Live-Action version, Usagi dressed as a bear and hid and there was no kiss. Okay, back to this episode--other changes are Luna telling the three their mission and also a brief thing about each of their personalities. Rei asks when will the Princess be revealed, Luna says when all five Senshi are present and a special block they have on her will be taken off.
I am not writing a simple summary and review this time as I am growing tired of the same thing over and over. We have seen this all before. They say this is closer to the Manga but I have been seeing the old Anime and it is all pretty much the same. Only major difference from this Anime and the original is that Naru is not involved in this version and that Rei and Ami actually go to the dance, but they pretty much serve the same purpose and make no difference to the Usagi and Mamoru plot.
What new thing is that Jadeite destroyed her Tiara and she got a new one this time. In the last Anime, it got destroyed and it magically came back the next time with no explanation. Sailor Jupiter premieres in two weeks.