Part Two begins with the story of the signature scientific achievement of our time: the mapping of the human genome. As scientists learn to read the genetic code, they grapple with the dangers inherent in increasingly sophisticated and easily available methods of intervening in the very essence of what makes us human, our DNA. 

Part One interweaves the present-day story of the Rosens, a young family on an odyssey to find a cure for their four-year-old daughter's rare genetic disease, with stories of the exciting discoveries of the early pioneers in genetics — Gregor Mendel, Thomas Hunt Morgan, Francis Crick, and James Watson. This episode also tracks the dark period in human history when a little genetic knowledge was used to justify terrifying human experiments that culminated in the Holocaust.