On a fog-bound London night, a soirée is taking place in the studio of artist Laurence Newtree. The guests include an eminent psychiatrist, a wealthy philanthropist and an observant young friend of Newtree’s, John Christmas. Before the evening is over, Newtree’s neighbour is found stabbed to death in what appears to be an impossible crime. But a mysterious man in a fez has been spotted in the fog asking for highly unlikely directions...

The resourceful John Christmas takes on the case, unofficially, leading to an ingenious solution no one could have expected, least of all Inspector Hembrow of Scotland Yard.