Deep in the darkest reaches of the East China Sea, the remote vessel Neptune edges through the utter blackness along the Ryuku Trench, guided by the steady hands of Richard and Robin Mariner. The Mariners are actually five thousand meters above Neptune. They are seated side by side in the virtual environment presented by the vehicle control room of Poseidon, testing their company Heritage Mariner's latest acquisition. Suddenly a distress call comes in. A submarine has become trapped on the seabed nearby and its crew are rapidly running out of air. The trapped submarine has been on news networks for the last few days as its widely publicized mission is to recover a fortune in gold lost aboard the greatest of Kublai Khan's huge treasure ships, which sailed for Japan in 1281 and vanished without trace. But as the Mariners and their associates guide Neptune to the rescue of the submariners trapped far below, a different, darker, story becomes plain. Was the submarine's voyage simply a cover for another, more sinister, mission?