The summer occupants of "Sunnyside" find the dead body of Arnold Armstrong, the son of the owner, on the circular staircase. Following the murder a bank failure is announced. Around these two events is woven a plot of absorbing interest.

Mary Roberts Rinehart was an American writer, often called the American Agatha Christie.Rinehart published her first mystery novel The Circular Staircase in 1908, which introduced the "had I but known" narrative style. Rinehart is also considered[by whom?] the source of "the butler did it" plot device in her novel The Door (1930), although the exact phrase does not appear in her work.