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Cruel Sea - broadcast in North America as Jurassic Beach - is the third episode of Walking with Dinosaurs.
This episode, like the previous episode, focuses on life in the late Jurassic period in Oxfordshire. The episode follows several Ophthalmosaurus pups and their fight against the elements. The episode also follows a male Liopleurodon, the largest carnivore ever. There are a few scenes concentrating on the small population of carnivorous dinosaurs in the region and how they are able to survive on such small areas. The episodes concentrates on the environment the inhabitants of late Jurassic Oxfordshire live in. Like the previous episodes, this episode emphasizes the Jurassic megafauna and their success.
The episode begins with an Eustreptospondylus ominously watching the water, looking for food. Then, suddenly, a massive Liopleurodon catches the dinosaur and plunges itself and its prey into the water and performs a death roll.
After the narrators tells the viewer about the continental shifts and how sea levels have risen, we see a trio of Cryptoclidus basking on the sea's edge. On land, they are cumbersome hulks but in the water, they are transformed. All of them drag themselves into the water and glide through the water. The camera then cuts to a colony of ammonites and jellyfish.
The camera then cuts to a pregnant, female Ophthalmosaurus who is being followed by hundreds more of her kind. The reason they are banding together is because they are about to give birth. One propels itself out of the water to breathe. Then, three Cryptoclidus pursue a blizzard of fish.
The episode then cuts to the shore where a flock of Rhamphorhynchus are fishing. One lands on a rocky structure and swallows its catch. As the Rhamphorhynchus continue fishing, two Ophthalmosaurus emerge from the water to breathe. The Ophthalmosaurus are going into labour. As one mother gives birth, a Liopleurodon lurks in the background. The newly born pup escapes its mother, it rushes to the water's surface to breathe. The pup re-enters the water and hides amongst the coral on the sea floor. It then cuts to a female Ophthalmosaurus trying to give birth to her cup but despite her struggles, the pup refuses to exit its mother. She stays near the water's surface so she can breathe. Unfortunately, her desperation attracts several Hybodus sharks. As the sharks home in, they are suddenly spooked and scared away by the Liopleurodon. The Liopleurodon darts towards the pregnant Ophthalmosaurus and attacks her. She is killed along with her pup. Her tail sank to the sea floor and was consumed by Hybodus. The Liopleurodon then swam away.
Some baby Ophthalmosaurus are seen within the coral reef. One leaves and investigates an ammonite. Despite his futile attempts, he tries to bite through its shell. The episode then cuts to an Eustreptospondylus swimming from one island to the other. When it walks onto dry land, he shakes off all the water. After walking through a forest, the Eustreptospondylus encounters another one of his kind who is feeding on an animal shell. The two carnivores fight over the carcass. The finder of the carcass is the victor and the loser left. The episode then cuts to several bark beetles liven within the trunk of a tree. Then, a Rhamphorhynchus penetrates through the bark and eats a grub.
The episode then cuts to a Hybodus pursuing a Ophthalmosaurus pup. The pup seeks refuge between some rocks. Then, the Ophthalmosaurus darts away so fast that the Hybodus cannot follow. A Cryptoclidus is then seen looking for pebbles and grit to swallow to counteract the air in its lungs. Several other Cryptoclidus are seen basking on the sea shore in the evening. During the night, several adult Ophthalmosaurus are seen hunting in the coral reefs for squid. With the approaching full moon, thousands of horseshoe crabs gather on the sea shore to lay their eggs, with the males competing to fertilise them.
In the morning, the Cryptoclidus re-enter the water and swim off. The flock of Rhamphorhynchus are then seen feeding on the eggs of horseshoe crabs. As they do so, an Eustreptospondylus attacks them, managing to kill one of them. It then manages to kill another one. Meanwhile, another Eustreptospondylus stalks a Rhamphorhynchus perched on a rocky cliff edge. However, the pterosaur heard it and flew away.
One month later, the horseshoe crab eggs hatch and are swept away into the sea. The next day, we see the Ophthalmosaurus pups. In the following week, they will be big enough to live outside the coral reef. Several Ophthalmosaurus are then seen chasing a shoal of fish. The episode then cuts to the Liopleurodon taking in air before returning to the depths. Unexpectedly, a female Liopleurodon invades his territory. This the bull will not tolerate. The male Liopleurodon confronts the female and attacks her by viciously biting one of her flippers. The female then leaves the area. Her wound attracts several Hybodus.
Four months after the pups were born, a storm brews and strikes the sea. The Ophthalmosaurus pups are buffeted by the storm but survive. After the storm passes, the total damage can be seen. Several Rhamphorhynchus lie dead and the bull Liopleurodon is left stranded on the land. The Liopleurodon's struggles catches the attention of several Eustreptospondylus. As the Eustreptospondylus start to gather, the Liopleurodon slowly dies. His enormous carcass is then feasted on by several Eustreptospondylus.
Most of the Ophthalmosaurus pups have survived the storm and now face life in the open sea. When the females return, they will be carrying the next generation.
In this third episode, the only track used is Cruel Sea, a 6:08 length soundtrack that brings together all the music piece of the episode (most of them, because all the pieces of musics of all the episodes aren't in the CD).