When Finn Kennedy comes to live in Dubai, he expects to do noting more adventurous than surf a few good waves. Then he meets Triton, the Sea Djinn, a supernatural being with spectacular powers. Triton reveals that Finn's parents have been kidnapped at sea by Hydrus, the evil Sea Djinn, who is preparing to wage war against Triton's and Finn's worlds in a battle of the Djinns that has been brewing for a thousand years.
Triton recruits Finn to his army and bestows on him some of his own powers. Finn, together with his cousin, Georgina, and his friend, Fred, prepares to travel to the underwater lair of Hydrus, far across the sea in the Dark Kingdom, to rescue his parents and fight Hydrus.
Along the way the friends meet Mr. Violet, no ordinary teacher, who trains them in the skills they will need if they are to have any chance of surviving.
Their training is cut short when they themselves in grave danger and must escape.
They steal a boat and seat sail for the Dark Kingdom...