Ask him how it all began, and he remembers the ice. It was a bitter morning in January, 1982, when Bernard Cathomas, aged thirty-six, carefully picked his way up a slippery, sloping Zurich street. His destination was No. 33, an ochre house with green shutters—the home of Heinrich Schmid, a linguist at the University of Zurich. Inside, the décor suggested that “professor” was an encompassing identity: old wooden floors, a faded carpet, a living room seemingly untouched since the nineteen-thirties, when Schmid had grown up in the house. Schmid’s wife served Rüeblitorte, a Swiss carrot cake that manages bourgeois indulgence...