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