Secret of the River by Fred M. White

Nevil lurked in the shadows at the top of the stairs until he had seen Jakes pilot the new-comer up to his room. He had ample time for reflection before his old servant returned. To begin with, he wondered how it was that the very man whom Murray and his associates had conspired to rob of a large sum of money should be the particular individual who had come down to Ashdown Croft for a week-end, apparently in the guise of a welcome guest, when he ought to have been at dagger's drawn with those three scoundrels.

Evidently there was some plot within a plot here--something that complicated the situation and rendered it more difficult of solution.