... that this is from A Reluctant Princess (not 
certain the ep. number) the episode right after 
Usagi/Serena is revealed as the Moon Princess. 
It's from a flashback and those two figures are 
the original Princess Serenity & Prince Endymion. 

Argh, I can't quite remember if its Luna/Artemis's 
flashback or Usagi/Mamoru's-- no wait, it's coming 
back to me... *dramatic music swells* It's from 
Usagi/Serenity's flashback. She starts screaming 
"Come back to me! Daaaaaarien!" somewhere around 
this point. (well, in the dub anyway ;)

And no, the bg isn't original but it sure is 
purty.