“I think so,” he said.  “Before I tell you who it is you must prepare yourself for a great shock.”

“I know who it is” she said—­“Mr. Holymead.”

There was no pretence about his astonishment.

“How on earth did you find out?”

She smiled a little at such a revelation of his appreciation of his own cleverness in having probed the mystery.

“I did not find it out,” she said.  “I had to be told.”

“And who told you, Miss Fewbanks?” he asked.  “Has he confessed to you?  How long have you known it?”

“I have known it only a few minutes,” she said.  “Will you tell me how you got on the track and all you have done?  I am greatly interested.  You have been wonderfully clever to find out.  I should never have guessed Mr. Holymead had anything to do with it—­I should never have thought it possible.  When you have finished I will tell you how I came to know.  The story is extremely simple—­and sordid.”

The fact that the key of the mystery had been in her hands only a few minutes was a solace to Crewe, as it detracted but little from the story he had to tell of patient investigations extending over weeks.

He pieced together the story of the tragedy as he had unravelled it.  Hill, he said, had conceived the idea of blackmailing her father after he had discovered the existence of some letters in a secret drawer of Sir Horace’s desk.  The fact that Sir Horace had kept these letters instead of destroying them as he had destroyed other letters of a somewhat similar kind showed that he was very much infatuated with the lady who wrote them.  That lady, as doubtless Miss Fewbanks had guessed, was Mrs. Holymead—­a lady with whom Sir Horace had been on very friendly terms before she married Mr. Holymead.

“What became of the letters?” asked Miss Fewbanks.  “Have you got them?”

“I think they are destroyed,” he said.  “Mrs. Holymead removed them from the secret drawer the day after the discovery of the murder.  She removed them when the police had charge of the house, and almost from under the eyes of Inspector Chippenfield.  It was a daring plan and well carried out.”

Miss Fewbanks heaved a sigh of relief on learning the fate of the letters.  It had been her intention to endeavour to obtain them if they were in Crewe’s possession, and destroy them.

Crewe explained that Hill was afraid to take the letters and then boldly blackmail Sir Horace.  The butler conceived the plan of getting Birchill to break into the house.  He did not take Birchill into his confidence with regard to the blackmailing scheme, but in order to induce Sir Horace to believe the burglar had stolen the letters he told Birchill to force open the desk, as he would probably find money or papers of value there.  But in order to prevent Birchill getting the letters if he should happen to stumble across the secret drawer, Hill removed them the day before.  His plan was to go to Riversbrook in the morning after