A romantic suspense thriller with espionage elements, the script was based on both a same-titled novel by Ronald Kirkbride, and the non-fiction book documenting the case of real-life double agent George Blake titled  The Springing of George Blake by Sean Bourke, the motion picture rights to both of which Hitchcock acquired.
A romantic suspense thriller with espionage elements, the script was adapted from both a novel by Ronald Kirkbride, called "The same-titled novel", and a non-fiction book about real-life double agent George Blake, titled "The Springing of George Blake" by Sean Bourke, which Hitchcock acquired the rights to.