Maddie, Will Treaty's fearless young apprentice, aided by Hal, Thorn, and the rest of the Heron Brotherband, tries to rescue her mother, Princess Cassandra, who is trapped in Castle Araluen.

On a mission to rescue Empress Justina's son from his pirate kidnappers, Hal and the brotherband crew set sail amid stormy seas to the island of Santorillos where they encounter a formidable obstacle, a caldera.