A BBC Horizon investigation into epigenetics, the idea that experience can change which genes switch on or off without altering the DNA sequence itself, and that those changes can pass to children and grandchildren. Researcher Lars Olov Bygren digs through nineteenth-century harvest and famine records from the isolated Swedish town of Overkalix, finding that a grandfather's access to food as a boy predicts his grandson's risk of early death generations later. Randy Jirtle's lab provides the animal evidence: genetically identical Agouti mice, born fat and yellow or lean and brown depending only on what their mothers ate during pregnancy, a visible demonstration of genes reading their environment. Geneticists and biologists interviewed on camera walk through how this challenges the standard picture of inheritance as a fixed code, raising the possibility that trauma, diet, and stress leave marks that outlast the person who experienced them. The film treats the science as unsettled rather than proven, laying out what the data shows and what remains speculation.