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.

Nigel Marven continues to search the ancient oceans for the deadliest sea monsters of all time. He must travel to the Eocene, 36 million years ago, and come face to face to the killer whale Basilosaurus. His next mission will take him to 4 million years in the past to tag the monster shark Megalodon, a shark that makes the Great White look like a minnow in comparison! But that's only if he survives his encounter with Dunkleosteus, the monster armored fish.

Nigel Marven continues his adventures in the ancient past. Diving into the Jurassic sea, Nigel risks life and limb literally against a pair of Liopleurodon, the massive pliosaur that dominated the Jurassic oceans. Nigel then travels to "Hell's Aquarium", the shallow sea that covered the American mid-west during the Cretaceous era. Here, he comes face to face with the monster mosasaurs, huge carnviourus reptiles with nasty attitudes. Of course, that's only if he survived his close encounter with the giant shark Megalodon.