In The Case of the Secret Inquisitors, Kin Yen was a brilliant Chinese aeronautical engineer who was negotiating with the British government for the development of his helicopter design. Subsequently he had disappeared. There followed intelligence reports that a foregin power had a helicopter of the Yen design on trial. Biggles later discovered that Yen had had his mind altered. The malevolent scientist Doctor Kahn had applied his mind manipulation techniques on Yen and had extracted from him the secret details of his helicopter design. Subsequently, Yen had been "programmed" to work as a mechanic at the sanatorium where Doctor Kahn managed his mind manipulation activities. Yen was rescued by Biggles towards the end of the story.