“Now you’ve hit the nail right on the head!” exclaimed Alleyn, gazing at her as if she was a sort of sibyl. “I felt assured of that. You know as well as I do, matron, that Sir Derek was a man with many bitter enemies. I may tell you in confidence that at the Yard we know where to look. We are in close touch with the Secret Service”—he noted with satisfaction the glint of intrigue in her eye—”and we are pretty sure how the land lies. In our midst—in our very midst, matron— are secret agents, secret societies, powers of evil known to the Yard but unsuspected by the general public. Mercifully so.” He stopped short, folded his arms, and wondered how much of this the woman would swallow. Apparently the whole dose.

“Fancy!” breathed Sister Marigold. “Just fancy!”

“Well—that’s the position,” said Alleyn grandly, throwing himself back in his chair. “But here’s my difficulty. Before we can fire point-blank we’ve got to clear away the other possibilities. Suppose we made an arrest now—what would be the defense? An attempt would be made to throw suspicion on innocent persons, on the very people who fought to save Sir Derek’s life, on the surgeon who operated, and on his assistants.”

“But that’s terrible!”

“Nevertheless it is what would happen. Now to meet that position I must have the actual history of Sir Derek’s operation, in all its details, at my fingers’ ends. That is why I have laid my cards on the table, matron, and that is why I have come to you.”

Sister Marigold stared at him so long that he wondered nervously if he had been inartistic. However, when she did speak, it was with the greatest air of earnestness.

“I shall consider it my duty,” she said, “to give you what help I can.”

Alleyn thought it better not to shake hands with her. He merely said with quiet reverence:

“Thank you, matron, you have made a wise decision. Now to come down to tin tacks. I understand Sir John performed the operation, assisted by Mr. Thoms and with Dr. Roberts as anæsthetist. Sir John gave the hyoscine injection and prepared it himself.”

“Yes. Sir John always does that. As I always say, he’s so conscientious.”

“Splendid, isn’t he? And Mr. Thoms gave the anti-gas injection. Nurse Harden brought it to him, didn’t she?”

“Yes, she did. Poor Harden, she was dreadfully upset. Sir Derek was a great friend of her own family, a very old Dorsetshire family, Mr.—er—”

“Really? Strange coincidence. She fainted afterwards, didn’t she, poor girl?”

“Yes. But I assure you she did her work all through the op., quite as usual—really.” Sister Marigold’s voice trailed away doubtfully.

“Someone said something about a delay over the anti-gas injection.”