Michael Trevelyan, an oceanologist, learns in London of the death on a remote Pacific atoll of his estranged brother Mark. Within a few days he has been followed, attacked and robbed of all his brothers' possessions save two things; a coded notebook and a single piece of rock. This proves to be the key to an immense natural treasure of minerals hidden somewhere on the Pacific seabed.
Setting sail with friends who are trained marine commandoes Trevelyan arrives in Tahiti to begin his search for the mineral hoard of which cobalt is the most valuable part. He must also investigate his brother's death, which he suspects is murder.
The search involves him and his allies in peril on land and at sea. They are hounded by a rival surveyor, pursued by vicious men who will do further murder to prevent one crime from becoming known, and arrested by the local authorities. Struggling to decipher the clues that will lead them to the cobalt deposits, Trevelyan must fight for survival with the woman he meets and loves. Finally the sea, having given up its secret, unleashes its own terrible power on the adventurers in a cataclysmic natural disaster.