Captain Arthur Hastings returns to England after an 18-month-long stay in Argentina. He intends to visit his old friend Hercule Poirot and is shocked to find Poirot about to leave for South America. Poirot wants to go to Rio de Janeiro where his newest client Abe Ryland awaits. The imminent departure of Poirot has to be postponed, however. An unexpected visitor called Mayerling comes in through Poirot's bedroom. There is no clue of how the man entered the upper floor apartment. Perhaps through the bedroom's window which is, however, at some distance from the ground. The man is covered from head to toes with dust and mud. He is emaciated as though he has been long imprisoned and with little access to food. He collapses on the floor. The only clue to what he wants is his saying Poirot's name (his only words) and writing the number 4, many times.