Not a moment was lost in following up the clue.  The house in question had been unoccupied for some time.  The owner was, however, known to Van Duyk, who at once called upon him.  He said that he had let it some weeks before, to a person who had stated that he was a merchant of Amsterdam, and intended to open a branch house at Dort.  He had paid him six months’ rent in advance, and had received the keys of the house.  He believed that some of his party had arrived, as he had himself seen two men go in, but the house was certainly not yet open for business.

Rupert, who had been all day at work following out other clues given by persons who had come forward, returned just as Mynheer van Duyk came back with the news.

“Thank God!” he said, “There is an end to uncertainty.  Your daughter is in that house, beyond all doubt.  It is only a question of action now.  Let us call in the burgomaster and the chief constable, and discuss how the rescue is to be effected.  It is probable that he has with him a dozen desperate fellows of his Black Forest gang, and the task of so arranging it that we may interpose between her and the arch-villain is a difficult one indeed.  While you send for these officials, I will go and reconnoitre the house; it is quite dark.”

The house differed little from its fellows.  It was old, with gables, and each floor projected beyond the one below it.  A dim light was visible in one of the upper rooms, while a far brighter light shone through the folds of curtains which had been drawn across a window lower down.  Rupert drew his own conclusions.

Returning, he found the burgomaster and chief constable already with Mynheer van Duyk.  After much discussion it was agreed that thirty picked men should be at Rupert’s orders at ten that night, an hour at which all Dort would already be sound asleep.

The chief constable then proceeded with Rupert to the houses situated behind that which was intended to be attacked.  It was reconnoitred from that side, and found to be in darkness.  The owners of these houses, strictly charged to secrecy, were informed of what was going on, and promised all aid in their power.  A dozen ladders of various lengths were now got together.

Then they went to the house adjoining, and made their way out on to the roof.  This, like many of the Dort houses, was furnished with a terrace, placed between the gabled roofs, which rose sharply on either side.  Here the owner, if disposed, could sit and smoke, and look on the river.  A table and benches were placed here, and a few tubs with shrubs and flowers.

A short, light ladder was brought up, and Rupert climbed up the steep roof, drew up his ladder, and descended on the other side.  The steep roof of the next house now faced him, and he was soon over this also, and stood on the little terrace of the house where he believed Maria was a prisoner.  It in all respects resembled that he had left.  The door leading to it appeared strong and firmly fastened.  He now retraced his steps.