A female Phosphorosaurus hunts lanternfish under the ocean's moonlit surface. Hesperornis hunt for fish until they themselves are hunted by Xiphactinus. A Mosasaurus hunts a group of Tuarangisaurus. Nostoceras hatchlings are at the mercy of the tides and juvenile Pyroraptor. The young Nostoceras that survived find themselves among Baculites and Diplomoceras in their new seagrass meadow home. A pod of Morturneria sift the muddy seabed in search of food.

Experience with David Attenborough the wonders of a lost world like never before in this epic series. Travel back 66 million years to when majestic dinosaurs and extraordinary creatures roamed the lands, seas, and skies.
In the first episode, a pregnant Tuarangisaurus is in distress and her young calf can sense it as she travels waters that are home to the ocean's deadliest predators.

The setting alternates between prehistory and modern day times in which scientists study the fossilized remains of the creatures in the film. In 2009, a group of paleontologists discover a rare fossil in Kansas. The fossil was previously exposed by a summer rain, and it appears to be a marine reptile, tracing back over 70 million years. It was a female Dolichorhynchops and will be the inspiration for telling the story of Dolly, who travels the Kansas Inland Sea, 80 million years ago during the late Cretaceous period with her family.
The film brings to life some of the most bizarre, ferocious and fascinating creatures to ever inhabit the ocean, incluing Tylosaurus, Xiphactinus, Cretoxyrhina, and Ammonite. It combines animation with recreations in a 3D prehistoric adventure. A journey to the bottom of the ancient oceans dramatizes awe-inspiring creatures.