One of the most fundamental human acts, sex has always been a mystery. This program examines aspects of reproduction, as well as what being male or female means at the cellular level. Dr. Roy Levin, a reproductive physiologist, uses an fMRI scanner to glean surprising images from a couple having intercourse. Startling physiological effects are seen over a mere six months in a woman who begins testosterone therapy. Experts include Marc Breedlove, Professor of Neuroscience at Michigan State University; Dr. Peter Goodfellow, renowned authority on the Y chromosome; Professor John Burn, clinical geneticist at Newcastle University; Dr. John Manning, one of the world’s leading specialists on testosterone; and Dr. Melissa Hines, Professor of Psychology at City University, who has studied testosterone’s influence on how children play. A BBC Production. (50 minutes, color)