midnight
I could only find the one weakness which seemed to be involved in Martin 's sitting up until 12.30 ; 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 .