The Valley of Fear

A murder at an English country estate is strangely related to a cipher message sent by an associate of Professor Moriarty

I have no doubt that what she heard was the report of the gun, and that this was
the real instant of the murder. If this is so, we have now to determine what Mr
Barker and Mrs Douglas, presuming that they are not the actual murderers, could
have been doing from a quarter to eleven, when the sound of the gun-shot
brought them down, until a quarter past eleven, when they rang the bell and
summoned the servants. What were they doing, and why did they not instantly
give the alarm?