Euterpe, the Greek muse of music, sets out to teach two siblings, Clara and David, the fundamentals of musical language. They must learn them quickly. if they want to free Apollo, the god of music and poetry, who has been kidnapped by Hades, owner and lord of the underworld. Some of the guardians of this place (Hydra, Medusa, Harpy, Chimera, Cerberus, Sphinx) will put the children to the test with different challenges at the end of each chapter. Clara and David have a difficult challenge ahead of them: defeat these terrible guardians of Hades to free Apollo and let music rule the world again.