"Lost World" is the first episode of Planet Dinosaur. This episode first broadcast on 14 September 2011.
The episode begins with John Hurt telling the story about the discovery of the first Spinosaurus specimens and the sheer size of the animal. The scene is set in North Africa 95 million years ago in a swamp where a lone Ouranosaurus walks alone. It calls for its herd which is feeding nearby. As it wanders through the swamp, a Spinosaurus lurks in the distance. As the Spinosaurus travels through the swamp, the Ouranosaurus is startled and runs away. Somewhat running for its life, it stops when it reaches the rest of its herd. As it looks around, it spots the Spinosaurus. As the theropod walks past the herd, they scatter even though they are not on its menu.
The narrator then talks about the distribution of the spinosaurids and the surprising oxygen isotopes levels found in Spinosaurus skull. The episode consequently cuts to a river where a crocodile is seen basking on the bank until the Spinosaurus arrives. The crocodile retreats to the water but is repelled by a shoal of Onchopristis. The Spinosaurus watches as the shoal swim past it. The narrator then goes into detail about Onchopristis' size and rostrum. The episode cuts to the river where the anadromous Onchopristis are seen migrating to shallow waters to breed. The Spinosaurus is seen walking on the edge of the river and then enters the river. The Spinosaurus continues watching its prey until it quickly catches a lone Onchopristis. The predator vigorously shakes its prey and then drops it and disables its helpless victim. It then begins eating the prey. Nearby, a Rugops sees the Spinosaurus feeding. As the Spinosaurus leaves the eaten prey to catch more Onchopristis, the Rugops approaches the prey and feeds on the leftovers.
The Spinosaurus lowers its snout in the water. The narrator explains Spinosaurus' various adaptations including pressure sensors on the tip of its snout. The scene continues and as the Onchopristis quickly swim past, the Spinosaurus waits until it catches another victim. The Spinosaurus tosses its prey onto the river bank where it tears and eats its caught meal. Seeing that the Spinosaurus is dangerously close, the threatened Rugops goes away and hides. As the Spinosaurus leaves once again to the river, the Rugops returns to feeding. The narrator goes into detail about the predator-prey relationship between Spinosaurus and Onchopristis. The episodes cuts the Spinosaurus swimming down the river.
The episode then cuts to a bone-ridden desert where suddenly, a large foot stamps onto the ground. The dinosaur is Carcharodontosaurus. The narrator then describes Carcharodontosaurus, its size and its territory. The Carcharodontosaurus is then seen roaring at a rival male which is challenging it for its territory. The pair confront each other until they charge at each other and wrestle. They both back down and then start attacking each other. They bite and rake each other with their claws until one backs down and admits defeat. The narrator then goes into detail about inter-species attacks and fights.
The episode then cuts to a forest where a herd of Ouranosaurus are seen feeding. In the distance, the victorious Carcharodontosaurus walks into the forest quietly. The narrator then goes into detail about the theory that Carcharodontosaurus had a complex system of air sacs similar to birds and its metabolism. The episode then cuts to the Carcharodontosaurus chasing the Ouranosaurus. As it closes in, the Ouranosaurus notices the Carcharodontosaurus and runs away. The Carcharodontosaurus follows quickly. The rest of the herd watches as its family member is chased. As the Carcharodontosaurus chases the Ouranosaurus out into the desert, it catches up and instantly wounds the its prey. The Ouranosaurus limps away and the Carcharodontosaurus follows. As it callospes because its wounds, the Ouranosaurus is quickly dying because of the Carcharodontosaurus. The killer then eats its prize.
The episode then cuts to the river where the Spinosaurus was hunting. Because of a severe drought, the Spinosaurus is forced to leave the river. However, it lingers there a little too long and a Sarcosuchus emerges from the river to warn the theropod away. The Spinosaurus leaves the river and finds a skeleton where some tiny pterosaurs are resting. The Spinosaurus huntes a pterosaur and eats it. The drought gets worse, however, and the Spinosaurus is forced to penetrate into Carcharodontosaurus territory. The two giant killers are eventually driven to confront each other over an Ouranosaurus carcass. There is a short but brutal fight between the two and it ends when the Spinosaurus claws the Carcharodontosaurus in the face. The Spinosaurus feasts on carrion as the Carcharodontosaurus backs away from it.
The Spinosaurus died because Carcharodontosaurus had bitten its sail in the fight. The Spinosaurus is an active hunter but therefore, when it had finished eating the dead Ouranosaurus, it starved in the drought. The injuries to its sail did not heal and the Spinosaurus died a few days later lying on the ground