midnight
But it would be physically impossible for the car -- a fifteen horse-power four-cylinder Northumberland , an average medium-power car -- to get to Southampton by half-past six unless it left Marlstone by midnight at latest . Motorists who will examine the road-map and make the calculations required , as I did in Manderson 's library that day , will agree that on the facts as they appeared there was absolutely no case against Marlowe . But even if they were not as they appeared ; if Manderson was dead by eleven o'clock , and if at about that time Marlowe impersonated him at White Gables ; if Marlowe retired to Manderson 's bedroom -- how can all this be reconciled with his appearance next morning at Southampton ? _ He had to get out of the house , unseen and unheard , and away in the car by midnight . _ And Martin , the sharp-eared Martin , was sitting up until twelve-thirty in his pantry , with the door open , listening for the telephone bell . Practically he was standing sentry over the foot of the staircase , the only staircase leading down from the bedroom floor . With this difficulty we arrive at the last and crucial phase of my investigation .