Marco and Anabella are brother and sister living with their parents and grandparents on Reunion Island in the Indian Ocean. The children's best friend is a dugong named Pons who lives in the shallow waters near their home. One summer day the serenity of island life is shattered by a powerful earthquake and the eruption of the volcano, Piton de la Fournaise. On this day, Marco has taken his friend Peter to a forbidden cave which becomes sealed by falling boulders during the earthquake. Ana reveals Marco's secret which leads Pons and the children's grandfather to attempt a daring undersea rescue.