The title of this episode and of episode The Unseen Hook (1981) were taken from G.K. Chesterton's short story "The Queer Feet" in which Father Brown says "I caught him, with an unseen hook and an invisible line which is long enough to let him wander to the ends of the world, and still to bring him back with a twitch upon the thread." This is the story (and the exact passage, in fact) that Lady Marchmain is reading when Sebastian made his dramatic interruption in episode 4, "Sebastian Against the World."