After meeting a water naiad named Minthe in the gloomy Underworld, Persephone feels especially grateful for the happy life she leads. In contrast, Minthe has to guard the stinky river Cocytus with no friends around!

When the students of Mount Olympus Academy are challenged to a geo-dashing competition on Earth, Persephone wants to win! But it turns out that Minthe envies Persephone. And when the competition takes the MOA team to the Underworld, Minthe shows her true colors and causes big trouble. Can Persephone figure out how to save her friends and become a friend to Minthe?