Nigel Marven searches the ancient seas for the most vicious animals to ever prowl the oceans. His first stop is the Ordovician, where he battles sea scorpions and encounters the giant orthocones. Next stop is the Triassic, where he tangles with Nothosaurs and must fend off the giant icthyosaur Cymbospondylus. His next stop could be his last, for he faces off with the armored fish Dunkleosteus.