The Murder at the Vicarage by Agatha Christie

There is little more to be told. Miss Marple's plan succeeded. Lawrence Redding was not an innocent man, and the hint of a witness of the change of capsule did indeed cause him to do "something foolish. " Such is the power of an evil conscience.

He was, of course, peculiarly placed. His first impulse, I imagine, must have been to cut and run. But there was his accomplice to consider. He could not leave without getting word to her, and he dared not wait till morning. So he went up to Old Hall that night -- and two of Colonel Melchett's most efficient officers followed him. He threw gravel at Anne Protheroe's window, aroused her, and an urgent whisper brought her down to speak with him. Doubtless they felt safer outside than in -- with the possibility of Lettice waking. But as it happened, the two police officers were able to overhear the conversation in full. It left the matter in no doubt. Miss Marple had been right on every count.