A few days before Christmas in 2008, Hervé Falciani was in a meeting at his office, in Geneva, when a team of police officers arrived to arrest him. Falciani, who was thirty-six, worked for H.S.B.C., then the largest bank in the world. He was on the staff of the company’s private Swiss bank, which serves clients who are wealthy enough to afford the minimum deposit—half a million dollars—required to open an account. Falciani had been at H.S.B.C. for eight years, initially in Monaco and then in Geneva. He was a computer technician who helped supervise security systems for the handling of client data. He had grown up in Monaco, where as a young man he had worked as a croupier at the Casino de Monte-Carlo, and developed an excellent poker face. As the Swiss police escorted him from the building, he insisted that he had done nothing wrong.