The sixth book in the adventure series by the most beloved children's author in the world. The four favorite friends, Jack, Lucy-Ann, Philip, and Dinah, are eagerly awaiting the summer holidays. They will go on a cruise in the Mediterranean, starting from Portugal and ending up in the Aegean islands. In Naples, Lucian, a fifteen-year-old boy with funny glasses and protruding teeth, will board. He will tell the group a strange story about a hidden treasure on a Greek island. The children are determined to reach the island of Rhenea, where they will experience an unparalleled adventure in underground tunnels, labyrinths, and catacombs, searching for the mythical treasure. What will the map reveal with the unknown, to the group, Greek words? Who is the strange Mr. Epikopoulos and what are his plans to seize the treasure? Are you ready for... adventure?