You have been brought there to personate someone, and the real person is imprisoned in this chamber.

That is obvious.

As to who this prisoner is, I have no doubt that it is the daughter, Miss Alice Rucastle, if I remember right, who was said to have gone to America.

You were chosen, doubtless, as resembling her in height, figure, and the colour of your hair.

Hers had been cut off, very possibly in some illness through which she has passed, and so, of course, yours had to be sacrificed also.