As soon as German agents were recruited to the British double-cross system they were quizzed about German espionage methods. In her interview Nathalie Sergueiew recounted the story of her life, described the appearance, character and relationships of her Abwehr contacts, and explained the unique cipher she was taught, which was based on the pages of a certain edition of a French novel. In this extract Sergueiew describes the methods of secret ink writing she learnt in Germany.