In the final pages of Valhalla Rising, Dirk Pitt discovered, to his shock, that he had two grown children he had never known-twenty-three-year-old fraternal twins born to a woman he thought had died in an underwater earthquake. Both have inherited his love of the sea: the girl, Summer, is a marine biologist; the boy, himself named Dirk, is a marine engineer. And now they are about to help their father in the adventure of a lifetime.