The love between a Lunarian and a Human is forbidden, as it was written over a thousand years ago. However, that would never stop Endymion or Serenity from seeing each other.

Their love was forbidden, they both knew that, but that will never stop them from seeing each other.

Lunarians are to never step foot onto Earth's soil, if so they will be punished. That is the rule set by the Kings of the Earth for over 100 years, however, that doesn't stop Princess Serenity from sneaking her way to Earth just to see Prince Endymion, and that doesn't stop Endymion from falling in love with her.