Neuroscientist Jim Fallon spends his career scanning the brains of psychopaths and serial killers, looking for the neural signature of a mind capable of murder. Then he runs his own family's brain scans as a control group and finds his own scan matches the pattern he has spent decades identifying in killers. The film, part of the BBC's Horizon strand, follows Fallon as he tests his genetics against his life story, asking what stopped him from becoming what his brain scan says he should be. Interviews with researchers push the question further, including a scientist who argues that oxytocin functions as a kind of moral molecule, shaping trust and empathy at a chemical level. Encounters with convicted killers sit alongside the lab work, putting faces to the data. The film treats nature and nurture as intertwined rather than opposed, using Fallon's own case to argue that genetics can load the gun while upbringing decides whether it fires, with real consequences for how the justice system judges intent and blame.