Inspector Head, having ascertained that Edward Carter has been shot down at his own door at four o'clock on a January morning, finds in the snow the murderer's footprints, leading to a gate, and stopping there! The tracks do not go on, nor do they reappear anywhere: the murderer, having walked as far as the gate, apparently vanished into thin air! This is the initial problem in a mystery into which is woven the love story of Hugh Denham and Marguerite...