Nigel travels back in time to the seventh deadliest sea of all time, the Ordovician period (450 million years ago), where he is attacked by sea scorpions and comes face-to-face with a giant orthocone. He then explores the sixth deadliest sea, the Triassic (230 million years ago), where he encounters a pair of Nothosaurus, the bizarre sea reptile Tanystropheus and the giant primitive predatory ichthyosaur Cymbospondylus, which Nigel keeps at bay through using a cattle prod. Nigel then dives in the fifth deadliest sea, the Devonian (360 million years ago). In the Devonian, Nigel encounters the strange primitive shark Stethacanthus and the placoderm Bothriolepis and he uses a rounded shark cage to protect himself from the giant predatory placoderm Dunkleosteus.

Leaving the Devonian, Nigel travels forward in time to the fourth deadliest sea, the Eocene (36 million years ago), where he encounters the strange elephant-related Arsinoitherium. Diving in the Tethys Sea, Nigel encounters the primitive whale Dorudon and the crew of The Ancient Mariner uses recorded whale sounds to attract the giant predatory primitive whale Basilosaurus. In the third deadliest sea, the Pliocene (4 million years ago), Nigel encounters the strange whale Odobenocetops and dives with a adolescent megalodon shark. He later uses the rounded shark cage to dive with an adult megalodon, the largest shark of all time.