Roger Sheringham is pleased to receive an invitation from Chief Inspector Moresby of Scotland Yard to accompany him to a murder scene. The top-floor flat at Monmouth Mansions has been burgled and the occupant, Miss Barnett, strangled. On an impulse Sheringham offers the dead spinster's niece, Stella Barnett, a job as his secretary. Unable to resist a mystery he then proceeds to theorise, believing he will find the murderer amongst the motley residents of the Mansions. Or will he?