Lorac, the pen name of the British writer Edith Caroline Rivett.

[1][2] It is the thirty-ninth in her long-running series featuring Chief Inspector Macdonald of Scotland Yard, one of the more orthodox detectives of the Golden Age of Detective Fiction.

[3] A young man recently arrived at Paddington Station on a train from Exeter is discovered with his head bashed in.

Any possible clues of identity have been taken from the body, and the assault was committed during a very heavy London Fog (specifically the Great Smog of London of December 1952).

With these very few leads to follow, Macdonald begins his investigation.