At five o'clock on the morning of the tenth of July, 1860, the front door of a certain house on Anchor Street, in the ancient seaport town of Rivermouth, might have been observed to open with great caution. This door, as the least imaginative reader may easily conjecture, did not open itself. It was opened by Miss Margaret Callaghan, who immediately closed it softly behind her, paused for a few seconds with an embarrassed air on the stone step, and then, throwing a furtive glance up at the second-story windows, passed hastily down the street towards the river, keeping close to the fences and garden walls on her left.