Deep in the East China Sea, the remote deep sea vessel Neptune edges through the blackness of the Ryukyu Trench. Richard and Robin Mariner are actually 5,000 metres above Neptune, in the vehicle control room of Poseidon, testing their company’s latest acquisition. Suddenly a distress call comes in from a trapped submarine that is running out of air. The submarine has been on news networks for the last few days as its widely publicised mission is to recover lost gold from the greatest of Kublai Khan’s huge treasure ships. But as the Mariners and their associates guide Neptune to the rescue a different, darker story is uncovered. Was the submarine’s voyage simply a mask for another, more sinister, mission…?