Tegan and the Green Moonstone follows the story of a young seamstress and cat creature, TEGAN, who learns her uncle has been kidnapped by the evil black rats, and knows the abduction is only part of the rodents' treaty violation with nearby clans, including her own. Tegan must now carry a sword and come to terms with her role in a prophecy to save neighboring clans threatened by the same black rats that have captured her uncle in a scheme to seize precious moonstones and control the kingdom.

With the help of her childhood friend and a pixie prince, Tegan explores a stately nation of whimsical animals, mythical creatures, magic gemstones, and terrifying sea monsters to rescue her uncle and ultimately prevent war.