Holmes. "When does he lunch?"

"About one, sir."

"Then Dr. Watson and I will come round in time. For the present, Mrs.
Warren, good-bye."

At half-past twelve we found ourselves upon the steps of Mrs. Warren's
house--a high, thin, yellow-brick edifice in Great Orme Street, a
narrow thoroughfare at the northeast side of the British Museum.
Standing as it does near the corner of the street, it commands a view
down Howe Street, with its ore pretentious houses. Holmes pointed with
a chuckle to one of these, a row of residential flats, which projected
so that they could not fail to catch the eye.

"See, Watson!" said he. "'High red house with stone facings.' There is
the signal station all right. We know the place, and we know the code;
so surely our task should be simple. There's a 'to let' card in that
window. It is evidently an empty flat to which the confederate has
access. Well, Mrs. Warren, what now?"

"I have it all ready for you. If you will both come up and leave your
boots below on the landing, I'll put you there now."

It was an excellent hiding-plate which she had arranged. The mirror
was so placed that, seated in the dark, we could very plainly see the
door opposite. We had hardly settled down in it, and Mrs. Warren left
us, when a distant tinkle announced that our mysterious neighbour had
rung. Presently the landlady appeared with the tray, laid it down upon
a chair beside the closed door, and then, treading heavily, departed.
Crouching together in the angle of the door, we kept our eyes fixed
upon the mirror. Suddenly, as the landlady's footsteps died away, there
was the creak of a turning key, the handle revolved, and two thin hands
darted out and lifted the tray form the chair. An instant later it was
hurriedly replaced, and I caught a glimpse of a dark, beautiful,
horrified face glaring at the narrow opening of the box-room. Then the
door crashed to, the key turned once more, and all was silence. Holmes
twitched my sleeve, and together we stole down the stair.

"I will call again in the evening," said he to the expectant landlady.
"I think, Watson, we can discuss this business better in our own
quarters."

"My surmise, as you saw, proved to be correct," said he, speaking from
the depths of his easy-chair. "There has been a substitution of
lodgers. What I did not foresee is that we should find a woman, and no
ordinary woman, Watson."

"She saw us."

"Well, she saw something to