The detective had been watching for four days before he realised suddenly that the house was inhabited.<br />&nbsp;<br />It was a sinister-looking house that stood alone upon a lonely shore in South Australia, and it lay by the margin of the waves in a little sandy cove between the dip of two high hills.<br />&nbsp;<br />It was a place where few men came, for it was cut off from the distant townships by long, barren wastes of rock-strewn land.<br />&nbsp;<br />There were no roads nor tracks within many miles of it, and its only highway was the dark and restless sea, forever teased and fretted by the winds that blew across the gulf.<br />&nbsp;<br />And for four whole days he had watched it through his binoculars from the cliff less than two hundred yards away, and the whole time there had been no suggestion about it of any life within.<br />&nbsp;It was a silent house, as still and silent as the grave.<br />&nbsp;Its door had never opened, he had seen no faces from its window and no smoke had ever issued from its chimneys--yet in the falling light of dusk that evening it had flashed to him, as lightning flashes through the blackness of a midnight sky, that human beings were in hiding there.<br />&nbsp;