Does light come in waves or particles? Experient will show either: the experimenter can choose. 'A double agent is like a trick of the light,' Kerner the physicist tells Blair the spycatcher. 'You get what you interrogate for.' Dual natures, of light and of people, are the theme of Tom Stoppard's espionage thriller. Kerner's secret research is being leaked to Moscow. Is Ridley the double? Or is Kerner a triple? Hapgood is the person to find out, and maybe it will need two of her.