Framed in the doorway of Poirot’s bedroom stood an uninvited guest, coated from head to foot in dust. The man’s gaunt face stared for a moment, then he swayed and fell. Who was he? Was he suffering from shock or just exhaustion? Above all, what was the significance of the figure 4, scribbled over and over again on a sheet of paper? We follow Hercule Poirot as he finds himself plunged into a world of international intrigue, risking his life to uncover the truth about ‘Number Four’.

Librarian's note #1: the concept of The Big Four first appeared as weekly short stories very loosely connected in 'The Sketch' in 1924. The 12 original stories were: 1) The Unexpected Guest, 2) The Adventure of the Dartmoor Bungalow, 3) The Lady on the Stairs, 4) The Radium Thieves, 5) In the House of the Enemy, 6) The Yellow Jasmine Mystery, 7) The Chess Problem, 8) The Baited Trap, 9) The Adventure of the Peroxide Blond, 10) The Terrible Catastrophe, 11) The Dying Chinaman, and 12) The Crag in the Dolomites. For her 1927 novel, Christie enhanced the linkages between the stories and shuffled them somewhat.