midnight
I could only find the one weakness which seemed to be involved in Martin 's sitting up until twelve-thirty ; and since his having been instructed to do so was certainly a part of the plan , meant to clinch the alibi for Marlowe , I knew there must be an explanation somewhere . If I could not find that explanation my theory was valueless . I must be able to show that at the time Martin went up to bed , the man who had shut himself in Manderson 's bedroom might have been many miles away on the road to Southampton . I had , however , a pretty good idea already -- as perhaps the reader of these lines has by this time , if I have made myself clear -- of how the escape of the false Manderson before midnight had been contrived . But I did not want what I was now about to do to be known . If I had chanced to be discovered at work , there would have been no concealing the direction of my suspicions . I resolved not to test them on this point until the next day , during the opening proceedings at the inquest .