The advent of horse riding was a momentous step in human history. But when and how did our ancestors first learn to master these animals? In a spectacular adventure, NOVA unlocks the mystery on the vast, grassy plains of Kazakhstan, where wild horses still roam free, and nomadic herders follow their traditional way of life. Investigating clues from archaeology and genetics, researchers reveal vivid evidence of the very first horsemen. They also discover warriors who swept across Europe and turn out to be the ancestors of millions today.