Stapleton's first idea was that this young stranger from Canada might possibly be done to death in London without coming down to Devonshire at all.
He distrusted his wife ever since she had refused to help him in laying a trap for the old man, and he dared not leave her long out of his sight for fear he should lose his influence over her.
It was for this reason that he took her to London with him.

They lodged, I find, at the Mexborough Private Hotel, in Craven Street, which was actually one of those called upon by my agent in search of evidence.

Here he kept his wife imprisoned in her room while he, disguised in a beard, followed Dr.Mortimer to Baker Street and afterwards to the station and to the Northumberland Hotel.

His wife had some inkling of his plans; but she had such a fear of her husband--a fear founded upon brutal ill-treatment--that she dare not write to warn the man whom she knew to be in danger.

If the letter should fall into Stapleton's hands her own life would not be safe.