Deposited by his mean parents in the care of his strange and sinister Grandad Jock in a remote farmhouse on the Scottish coast, Jimmy Black is far from happy. Mere unhappiness turns to abject horror when he is force-fed the saltiest porridge in the history of the world each morning. To devise a plan to get rid of the foul mixture seems simple: little does Jimmy realize that doing so will catapult him into a hair-raising adventure deep beneath the ocean floor and that the very survival of the human race will come to rest upon his quivering shoulders!