midnight
`` I think I know who the real Jane Finn is , and I 've even got an idea where the papers are . That last 's only a guess , of course , but I 've a sort of feeling it 'll turn out right . Anyhow , I enclose it in a sealed envelope for what it 's worth . I 'm going to ask you not to open it until the very last moment , midnight on the 28th , in fact . You 'll understand why in a minute . You see , I 've figured it out that those things of Tuppence 's are a plant too , and she 's no more drowned than I am . The way I reason is this : as a last chance they 'll let Jane Finn escape in the hope that she 's been shamming this memory stunt , and that once she thinks she 's free she 'll go right away to the cache .