The book describes the adventure of two boys, Alex and Theo, who discover a mysterious path on the island of Thassos using an old map. They encounter mythical creatures such as a satyr, a nymph and a centaur who teach them important lessons about compassion, wisdom and courage. The journey ultimately takes them to the underwater world, where they come into contact with mermaids and other sea creatures and find treasure. Ultimately, they return to their village enriched with wisdom and friendship to share their knowledge.