In a special department of the Ministry of Protection, hush-hush work is proceeding on the Theta Screen, a secret process which will revolutionize the aircraft industry. The secret is shared by Sir James Bird, Peter Faber, Captain Knowles and Mr Noote. Following a quarrel, Sir James loses control and orders Peter Faber to leave. Alone in the office late at night, Sir James is shot by an unseen hand. Detectvie-Inspector Richards begins the investigation to unmask the murderer.