The first episode begins with an illustration of the giant impact hypothesis: approximately 4.4 billion years ago when the Earth was formed, it is conjectured that a planet-like object referred to as Theia collided into the early Earth, dynamically reshaping the Earth and forming the moon. The episode then jumps ahead to the Cambrian Explosion, showing the first diversification of life in the sea.

The reptiles evolve, and the race for survival continues on land. Giant arthropods lurk in the Carboniferous forest, and sail-backed mammal-like reptiles fight for the survival of their young in the Permian.