Sharkey was born on a fortunate tide, and everyone on the giant submersible Rampart knows it. He's a hero, a future admiral, beloved by the ancestors. The trouble is, it's all based on a lie, and now Sharkey is thirteen, the whole thing has begun to fall apart. He's been a fake hero for years, but when tragedy strikes, he must become a real one. And he has no idea how to go about it... Meanwhile above water, Petrel, Fin and the crew of the Oyster are on a mission, a mission to defeat the Devouts and bring knowledge back to the world, a mission they have no idea how to carry out. When the Captain of the Oyster comes up with a plan, Fin and Petrel have no choice but to accompany him - no matter how ridiculous or how dangerous the plan seems ... Can the Sunkers and the Ice Breakers put aside their differences and work together? Or will the Devouts finally catch up with them all?