The adventure continues as Cruz Coronado and his fellow explorers are jolted from their normal routines at Explorer Academy's classroom-at-sea and thrust into survival mode in the fifth book in the thrilling fiction series.

On an island nation in the Indian Ocean, the explorers venture through a vast underwater world and take part in a friendly robotics competition. But the tides of good fortune change quickly. Suddenly, Team Cousteau jumps into action to rescue a faculty member from the brink of death. Meanwhile, the team follows the cipher's clues to a magnificent tomb and centre of spirituality precariously perched on the side of a cliff. There, Cruz is confronted by a familliar foe who's determined to stop him from completing his most important mission of all: Retrieving the final pieces of his mother's cipher. In this life-or-death showdown, Cruz experiences the ultimate sacrifice and uncovers a hidden message that makes him question his own mortality.