Fossils buried in the Australian outback rewrite what scientists thought they knew about the continent's prehistoric past, and this episode of NOVA's series follows host and paleontologist Richard Smith as he tracks down the reptiles that ruled the land and sea. In the dry interior, researchers excavate the remains of an ancient inland ocean, where opal has preserved the bones of marine reptiles in vivid color. Long-necked plesiosaurs surface again and again, hunting shelled prey that grew as large as truck tires, and Kronosaurus emerges as the apex predator of this vanished sea, its skull measuring twice the length of a Tyrannosaurus rex's. On land, Smith examines giant dinosaur trackways and skeletal finds that are only now filling in Australia's fossil record, long considered one of the sparsest in the world. Mammals get their moment too: the platypus already existed alongside these reptiles, an odd survivor waiting out the age of monsters. The film moves between desert dig sites and reconstructed creatures to piece together a distinctly Australian chapter of the Age of Dinosaurs.