It posits a speculative evolution of dragons from the Cretaceous period up to the 15th century, and suppositions about what dragon life and behavior might have been like if they had existed and evolved. It uses the premise that the ubiquity of dragons in world mythology suggests that dragons could have existed. They are depicted as a scientifically feasible species of reptile that could have evolved, somewhat similar to the depiction of dragons in the Dragonology series of books. The dragons featured in the show were designed by John Sibbick.
The program switches between two stories. The first uses CGI to show the dragons in their natural habitat throughout history. The second shows the story of a modern-day scientist at a museum, Dr. Tanner, who believes in dragons. When the frozen remains of an unknown creature are discovered in the Carpathian Mountains, Tanner, and two colleagues from the museum, undertake the task to examine the specimen to try to save his reputation. Once there, they discover that the creature is a dragon. Tanner and his colleagues set about working out how it lived and died.
The docufiction features two interwoven stories. Jack Tanner, an American paleontologist working for the Natural History Museum in London, suggests the theory that a carbonized Tyrannosaurus rex skeleton on display was killed by a dragon, causing him to believe that the legends were more than myth. This ruins Tanner's reputation. As viewed in a flashback, Tanner's theory is proven true, as said Tyrannosaurus battles a female dragon in the Cretaceous but is mortally wounded. The female dies from her wounds, forcing her infant to survive on his own, escaping a male dragon by learning how to fly for the first time. A later vignette shows the dragon, now an adult, trying to mate.
The museum is contacted by Romanian authorities, who discovered an alleged corpse of a dragon in the Carpathian Mountains, along with two carbonized human bodies from the 15th century. Tanner and two colleagues are sent to examine the bodies, moved to a warehouse. The scientists are baffled by the corpse, discovering despite being 900 pounds (410 kg), it was capable of both flight and breathing fire by storing bacteria and hydrogen inside its body. Marine Dragons survived the KT extinction, before evolving into other subspecies, such as a Chinese forest dragon, able to glide on its smaller wings and capable of camouflaging itself. The Forest Dragon hunts a wild boar and Bengal tiger, but the arrival of humans in the forest challenges its survival.
By analyzing the dead dragon's reproductive system, Tanner concludes the corpse is that of an infant, killed by the humans. The scientists travel to the mountains to explore the caves where the corpses were found. In 1475, a lone female dragon on the verge of extinction lives in the Carpathian Mountains, looking for a mate. A male arrives from the Atlas Mountains and they perform an airborne courtship ritual, free falling from the sky at high speed. Tanner discovers a preserved dragon egg in the cave. It is revealed that the male dragon guards the nest, made from a cluster of rocks and the eggs are kept warm for preservation. However, the male is negligent, letting one of the eggs die, and is chased away by the female.
Sometime later, the dragon has had a lone daughter, hunting sheep from the local shepherds who hired dragon slayers to kill them if they manage to get too close. A hunter and his squire attack, slaying the daughter but are in turn killed by the mother. Tanner discovers more human corpses and then that of the female dragon, twice the size of the infant. In a final flashback, a larger group of hunters approach the cave, leading to the deaths of all involved. Tanner and his team take the dragons to the museum, reuniting mother and daughter. A year later, Tanner receives information of another discovery and rushes off to investigate. What he will find remains unknown.
The Scotsman opined that The Last Dragon's computer graphics made it "awesome", but ultimately the show gave the feeling of conveying the message "Do not believe this slice of old hokum" to the viewer. According to The New York Times "it's easy to forget that [the film] isn't a serious documentary" after the fiction disclaimer at the beginning, judging the computer graphics to be well made, sometimes beautiful, but not impressive "to the point of wonder".