The Man in Black by Stanley J. Weyman

When Jehan, in a fever of indignation, slipped stealthily out of the house in the Rue Touchet and sped up the dark, quiet street after Madame de Vidoche, he had no subtler purpose in his mind than to overtake her and warn her. The lady had spoken kindly to him on the night of the supper at Les Andelys. She was young, weak, oppressed; the plot against her seemed to the child to be fiendish in its artfulness. It needed no more to rouse every chivalrous instinct in his nature -- and these in a boy should be many, or woe betide the man -- and determine him to save her.

He thought that if he could overtake her and warn her all would be well; and at first his purpose went no farther than that. But as he ran, now looking over his shoulder in terror, and now peering into the darkness ahead, sometimes slipping into the gutter in his haste, and sometimes stumbling over a projecting step, a new and whimsical thought flashed into his mind, and in a moment fascinated him. How it came to one so young, whether the astrologer's duplicity, to which he had been a witness, suggested it, or it sprang from some precocious aptitude in the boy's own nature, it is impossible to say. But on a sudden there it was in his mind, full-grown, full-armed, a perfect scheme. He had only a few minutes in which to consider it before he caught madame up, and the time to put it into execution came; but in that interval he found no flaw in it. Rather he revelled in it. It satisfied the boy's stern sense of retribution and justice. It more than satisfied the boy's love of mischief and trickery.