In the first episode, we see Australopithecus afarensis, and focus on their evolved bipedality (walking just on feet, our legs). More specifically, the story follows the famous Lucy and her relatives, as they first develop a leadership conflict following the death of the alpha male due to a crocodile attack, and then are attacked by a rival troupe. The attack ends with death of Lucy herself, and her eldest daughter caring for Lucy's now-orphaned baby (her sibling), as a sign of the developing humanity in these "apemen".
The second episode leaps forward to a time when Paranthropus boisei, Homo habilis and Homo rudolfensis coexist. H. habilis is depicted as an intelligent carnivore that is more adaptable than the herbivore P. boisei. The two species are contrasted, with H. habilis being "a leap foward", while P. boisei are "a master of herbivores" - i.e. they are specialized herbivores while H. habilis are generalized carnivores. Consequently, though P. boisei are able to eat termites, tall grasses and hard acacia pods in difficult times, they will not be able to survive in the future, when at the beginning of the next Ice Age the climate will change, and these plants will be gone for good. H. habilis, on the contrary, have become smart by eating carrion and bone marrow among other things, and evolving a basic social behavior, which is more firm than that of P. boisei, will continue to survive, until it evolves into Homo ergaster, seen in the next episode, who has developed these traits to a greater extent. The episode also briefly shows the H. rudolfensis, remarking that albeit they are taller, they are very similar to the H. habilis.
In the third episode, Homo ergaster is depicted as the first creature to master the art of tracking. This was made possible because their diet has grown increasingly more carnivorous, and the nutrients in meat made them even smarter than H. habilis in the last episode. They also begin to form into tribal societies, with genuine bonds between their men and women, though violence is still occurring. The episode later shows H. ergaster spreading into Asia, becoming Homo erectus and encountering the enormous herbivorous ape Gigantopithecus, a.k.a "the original King Kong". However, for the next million years, H. ergaster is still very much of an animal, following its instinct, but then, they are shown harnessing fire and beginning to break-away from their direct dependence on their environment. (This ties neatly into the next and final episode, which is centered on human mind and imagination.)
The fourth episode talks about the mental evolution of the humans, as opposed to the physical in previous ones. First we leap forward to a time when Homo heidelbergensis is living in Great Britain. H. Heidelbergensis is depicted as intelligent and sensitive but lacking in the ability to comprehend an afterlife, or anything that isn't in the "here and now". Next, the episode shows a life of a Neanderthal clan, how they lived and hunted wild animals, including the mighty mammoths during the last Ice age. They are intelligent but still lack the imagination of modern humans. Finally, we see modern humans (represented by Bushmen) in Africa, who had to become imaginative and inventive to survive the long drought, and finally glimpse the cave painters of Europe, who had evolved the idea of the afterlife and the supernatural, and who are now ready to start the human history as it is now known (and drive-out the Neanderthals to extinction).