"If one does not make the move, then the other must; by allowing the opponent to attack, you can learn something about them."

An uninvited guest stands at the door of Hercule Poirot's bedroom. He looks exhausted, covered in dust from head to toe. He fixes his tormented gaze on the famous detective for a moment and then collapses to the floor. Who is he? Why is he in a state of shock? And, most importantly, what does the number 4 mean that he writes over and over with a trembling hand on a sheet of paper?

Poirot will find himself entangled in an international intrigue and will risk his own life to solve the mystery of the number 4...