One cold winter morning on December 17th last year, a small blue car stopped on a bridge in a provincial town in northern Germany. Three men dressed in 10 heavy black coats got out and stood on the bridge. While they waited there, they kept on looking over the side. Fifteen minutes later, a motor-boat sailed past and drew up by the river-bank. Three men got out of the boat and looked up at the bridge. The men on the bridge silently walked down the stone steps leading to the river-bank. No words were spoken when they met the men from the boat. 15 After a while, the motor-boat moved off and three men returned to the bridge. Now, only two of them were wearing black coats. The third was dressed in a light grey jacket. Anyone who had been watching the scene might not have realized that two master spies had been exchanged on that cold winter morning.