The Saint wasted no time on those, for he was without the necessary instrument to force the catch of a window, and he knew that front doors are invariably solid. Back doors, on the other hand, he knew equally well, are often vulnerable, for the intelligent foresight of the honest householder frequently stops short of grasping the fact that the best-class burglar may on occasion stoop to using the servants' entrance. The Saint accordingly edged round the side of the house, Patricia following him.

They walked over grass, still damp and spongy from the rain that had deluged the country for the past six days. The humming of the dynamo was now unmistakable, and with it could be heard the thrum and whir of the motor that drove it. The noise seemed, at one point, to come from beneath their feet.

Then they rounded the second corner, and the Saint halted so abruptly that Patricia found herself two paces ahead of him.

"This is fun!" whispered the Saint.

And yet by daylight it would have been a perfectly ordinary sight. Many country houses possess greenhouses, and it is even conceivable that an enthusiastic horticulturist might have at­tached to his house a greenhouse some twenty-five yards long, and high enough to give a tall man some four feet of head­room.

But such a greenhouse brightly lighted up at half-past eleven at night is no ordinary spectacle. And the phenomenon becomes even more extraordinary—to an inquisitive mind like the Saint's—when the species of vegetable matter for which such an excellent illumination is provided is screened from the eyes of the outside world by dark curtains closely drawn under the glass.

Simon Templar needed no encouragement to probe further into the mystery, and the girl was beside him when he stepped stealthily to a two-inch gap in the curtains.

A moment later he found Patricia Holm gripping his arm with hands that trembled ever so slightly.