Detective Sergeant Kathleen Doyle has been called-in to assist with a few unsound-mind murders, lately-murders committed by a person who appears to be mentally unhinged. This type of murder is always a concern for Scotland Yard, since it raises the possibility that there's a serial killer on the loose. And it doesn't help matters that the weather is so very cold and miserable-small wonder, that these killers have gone off the deep end; Doyle was half-way there herself, what with Christmas coming far too quickly, and a husband who was showing some troubling signs of his own.
