Plummer’s two plain-clothes men and I reached the neighbourhood of the bank with a quarter of an hour to spare, or rather more. We dismissed the cab at some little distance from the spot, and approached singly, so that it was not difficult for us to slip in separately among the dozen or fifteen clerks as they arrived. We passed directly into the manager’s room, the door of which opened into the space left for the public before the counter. From this room the whole of the outer office was visible through the glass of the partition. The manager, Mr. Blockley, a quick, intelligent man of thirty-six or so, gave us chairs and pointed out how best we could watch the counter without ourselves being observed.

“If a letter is sent,” he said, “it will be brought here to me, of course, and I will bring the messenger in. If a cheque is presented from Mayes, I have told the cashier to slide that big ledger off his desk accidentally with his elbow. That will be your signal, and then you can do whatever you think proper. I don’t think I can do any more than that.”

We took our positions and waited. I felt pretty sure that if Mayes sent at all it would be early, for obvious reasons. And I was right, for the very first customer was our man.

He stepped in briskly scarcely a minute after the manager had ceased speaking, and I remembered having seen him waiting at the street corner as I came along. He was a well-dressed, smart enough looking man, in frock coat and tall hat. He took a letter-case from his pocket, picked out a cheque from the rest of the papers in it, and passed it under the wire grille of the counter.

The cashier took it, turned it over, and shifted mechanically to post the amount in the book on his desk. As he did so his elbow touched the heavy ledger which the manager had pointed out to us, and it fell with a crash. The cashier calmly put his pen behind his ear, and stooped to pick up the book, but even as he did it the two Scotland Yard men were out before the counter, and had sidled up to the stranger, one on each side.

“May we see that cheque, if you please?” asked one, and the cashier turned its face toward him. “Ah, just so; a hundred pounds — Mayes. We must just trouble you to come with us, if you please. There is some explanation wanted about that cheque.”

I had followed the two men from the manager’s room, and now I saw that while one had laid his hand on the stranger’s shoulder the other had taken him by the opposite arm. “Why,” said the former, looking into his face, “it’s Broady Sims!”

“All right,” the man growled resignedly. “It’s a cop. I’ll go quiet.”

But as he spoke I saw the free hand steal out behind him and pitch away a crumpled fragment of paper. One of the policemen saw it too, followed it with his eyes, and saw me snatch it up.

“That’s right, sir,” he said, “take care of that; and we’ll have a cab, in case anything else drops accidentally. It’s just a turning over, Broady, that’s what it is.”

I spread out the piece of paper, and was astonished to find inscribed on it just such another series of figures, in groups of eight, as was found in the cypher message in the Case of the Lever Key.

Here was a great find — a secret message as clear to me as to Mayes himself, and as likely as not the scrap of paper that would hang him! I took one of the plain-clothes men aside while the other kept his hold of Broady Sims.

“This is very important,” I said. “It is a cypher message which Mr. Hewitt can read — or I, myself, in fact, with a little time. Must you take it with you? If so, I’ll make a copy now.”

“Well, sir, we’re responsible, you see,” the man said, “so I think we must take it; so perhaps you’d better make a copy, as you suggest.”

“Very well,” I said, “that is done in a few seconds. You can take your man off, and I will go direct to Mr. Hewitt and Inspector Plummer with the copy.” And with that I made the copy, which read thus:—

23, 19, 15, 1, 9, 14, 9, 2; 20, 8, 1,
20, 14, 14, 20, 8; 14, 5, 12, 4, 9, 7,
5, 14; 3, 8, 18, 23, 0, 14, 1, 8; 22,
9, 6, 1, 18, 3, 5, 1; 19, 14, 15, 21,
9, 0, 20, 12; 18, 12, 21, 1, 6, 23, 20,
12; 9, 18, 15, 5, 18, 13, 12, 20.

It struck me to ask the manager if the cheque just presented were one of those procured from Mr. Trenaman the night before, and I found that it was. Then I left the policemen with their prisoner and made for the nearest cab-rank. This cypher message, no doubt conveying Mayes’s instructions to the man just captured, was probably of the utmost importance, and Hewitt must see it at once; and as the cab ambled along towards Barbican I busied myself in deciphering the figures according to the plan of the knight’s move in chess, as Hewitt had explained to me. I could only see two noughts among the numbers, so plainly it was a longer message than the one then deciphered — one of sixty-two letters, in fact. I turned the figures into the letters corresponding in the alphabet, a for 1, b for 2, and so on, as Hewitt had done, and I arranged these letters in the squares of a roughly drawn chessboard, so that they stood thus:—

w s o a i n i b
t h a t n n t h
n e l d i g e n
c h r w o n a h
v i f a r c e a
s n o u i o t l
r l u a f w t l
i r o e r m l t

The letters thus set out, to read off the message was a simple task enough, in view of the key Hewitt had given me. I began, as in the case of the Lever Key message, at the right-hand top corner, and taking the knight’s move from b to e in the last square but one of the third line, thence to a at the end of the fifth line, and so to t in the seventh line, and from that to r (fifth square in bottom line), u in seventh line and so on, in the order shown by the Lever Key message, a copy of which I kept as a curiosity in my pocket-book. So I read the message through, and I set it down thus:—

Be at ruin Channel Marsh to-night twelve; wait in hall for instruc. Word final.

The general meaning of this seemed clear enough. The man whom the policeman had recognised as Broady Sims was to be at some spot — a ruined building, it would seem — in a place called Channel Marsh, at midnight, there to wait in the hall for instructions; no doubt for instructions where to take the hundred pounds he was to have got from the bank. “Word final” was not so clear, though I judged — and I think rightly — that it meant that the word “final” was to be used as a password by which the two messengers should know each other.

I was almost at my destination, and was cogitating the message and its meaning, when the cab checked at some traffic in Barbican, just by the “Compasses” public-house, and Mr. Victor Peytral hailed me and climbed on the step of the cab.

“I was just going to see ............