The Aldens have joined Dr. Iris Perez as she investigates legendary creatures around the world. In this adventure, the children investigate what might be living in the deep trench off the coast of Puerto Rico. From manatees to mermaids to sea monsters, creatures of the sea have been attracting tourists to the area for years. But when a man on a nearby island goes missing, it's up to the Aldens to sort out fact from fiction.