"Her knowledge was a threat to these men; therefore her memory was deliberately destroyed. How, we do not yet know. But they stopped short of murder, or at least the leader did.

"Then one day she manages to throw off her drugged stupor long enough to manufacture an escape. She falls almost at once into the hands of the police. Do they at once take steps to retrieve their captive? You saw how easily it could be done. But no, they wait two days.

"I think there must have been a division in their ranks. The first decision was to let her go. After all, the trouble of keeping her a prisoner must have been a great one. The decision was made by the one whose belief in the permanency of her loss of memory was secure: the doctor. But Sacker, who had feared her from the first, was not convinced. As soon as an opportunity arose to evade his dominant companion, he made a unilateral expedition to eliminate the threat of her testimony once and for all. We know what happened then.

"How criminals are so often their own worst enemies," he went on philosophically. "By Sacker's very attempt to make himself secure, he has drawn all our attention upon them and given us the leads necessary to continue our case. If he had let well enough alone, she would probably have spent some time institutionalized as an unclaimed madwoman whose testimony would be discounted by all around her. The dominant partner, the doctor, had the intelligence to foresee this and the self-control to stand back and let it happen. I predict it is going to be a pleasure doing business with that gentleman, Watson; intelligence has been a quality lamentably lacking in the criminal classes ever since the removal of the late Professor Moriarty.

"Well, I now must be off to question the second cabman, if he hasn't taken to his heels in dismay by now. I gave him a few shillings and sent him around the corner to wait at the nearest pub. Then I must catch up with Lestrade and apprise him of this new turn of events. If you've no objection, I'd like to leave my client here under your medical care. In fact, if no one else turns up with a more legitimate claim to her, I should like you to take her on as a resident patient. If it won't incommode your household too much," he added as a palpable afterthought.