There was nothing very remarkable about the middle-aged couple who lived in the low, stone-clad villa on calle Padre Cardiel, a quiet residential street in the leafy Parque Luro district of Argentina’s best-known seaside town, Mar del Plata. Patricia Kadgien, 58, was born in Buenos Aires, five hours to the north. Her social media described her as a yoga teacher and practitioner of biodecoding, an obscure alternative therapy that claims to cure illness by resolving past traumas. Her husband, Juan Carlos Cortegoso, 61, built and raced go-karts. Like many in this neighbourhood, the couple were comfortably off, and discreet. “Patri...