One of my all-time favorite episodes. Luna Luna, the space princess, has landed on Earth near Mystery Lake. Under the full moon, she can undergo the remarkable transformation into a queen. When her brother Dodo encounters the evil hunter Villainous, the influence of "human brainwaves" (!?!?) overwhelm the simple minded alien. The wicked pair plot to sell the Princess to the circus and ensure Dodo's ascent to the throne.
Astroboy tells Doctor Elefun:
"The waters of Mystery Lake contain the secret ingredient of Moon Mist. When the princess bathes in the mist, she can transform into a queen and return home."