After their yacht capsizes in a storm off the coast of Portugal, Andrew and his fiancee Amy awaken strapped to stainless-steel tables inside a stone fortress miles below the ocean's surface — though they have no idea where they are or how they survived. Next, a frog-colored creature with webbed fingers steps out of the shadows and begins speaking to him. After recovering from the shock of its appearance, the creature realizes Andrew speaks English and, introducing himself as a doctor, promptly begins treating the bump on Andrew's head.
After that, Andrew learns they are in the sunken Kingdom of Attea and that the great seer Telemus foretold their ship's sinking before he died, warning only that the fate of the kingdom depended on their rescue. He and Amy are then told that, knowing of Attea's existence now, they may never leave and must either become Atteans or die by sea-snake venom.
Having no choice, Andrew takes the oath and is given his new bio-suit so he can endure the pressure at this depth. When Andrew's suit changes color, everyone in the room celebrates, shaking hands and patting each other on the back, though they refuse to tell him why.
After that, Andrew and Amy are taken to a large suite within the fortress, where Nestor, Attea's senior physician, explains the markings on Andrew's bio-suit and reveals that Andrew is the rightful king of Attea. After snorting his wine out his nose, Andrew protests, insisting he is from Maine, as were his parents, grandparents, and great-grandparents. Nestor then informs him that the only logical explanation is that Andrew is the great-grandson of Princess Helena, who married the sea captain who rescued her after her glide-sub was badly damaged in a storm.
Then they learn about the royal family and Andrew's murderous cousin, Prince Helios, who will stop at nothing to make sure Andrew and Amy never reach the palace and assume the throne. Prince Helios's spies are everywhere, and it's rumored he is the secret leader of the Wild Ones, the enormous band of criminals who hide in the cracks and corners of the kingdom, only coming out to loot and plunder.
With Andrew disguised as one of the archers and Amy disguised as a medic, they set out on the perilous twelve-day march across the kingdom to the palace with two hundred of Captain Ajax's best men and the fate of all Attea hanging in the balance.
But news of Andrew's existence is too big to contain and soon leaks out, and Prince Helios tries to have them assassinated before unleashing the full force of the Wild Ones on their troupe. They must now fight their way to the east cavern and the palace.
