1963 in Berlin. A morning spy exchange amid the height of the Cold War. Martin Keller, an American physicist who once made headlines but later vanished into the English jail system, is on one side of the exchange. His American passport was Keller's most important possession. Keller's deepest need was to visit his small kid and ex-wife Sabine. Martin, though, has inquiries. Who requested him? Who was the deal's broker? Just one KGB agent was brought home by the KGB? Or perhaps a more intimate intervention, as he hopes? He has been with the organisation long enough to understand that nothing happens by accident. He is wanted by them for a reason. Not physics; his knowledge is out-of-date.? Something else, which he cannot learn until he arrives in East Berlin, when suddenly the game is afoot.