The twelve stories all feature an unusual detective team: Mr. Satterthwaite, a bachelor in his sixties who has spent his life as a spectator not a participant, and the mysterious Mr. Harley Quin, who appears unexpectedly and disappears as suddenly. Mr. Quin provides Mr. Satterthwaite with insight through carefully worded questions and observations that lead Mr. Satterthwaite to the answers.
The stories in this short story collection are: The Coming of Mr. Quin, The Shadow on the Glass, At the Bells and Motley, The Sign in the Sky, The Soul of the Croupier, The World's End, The Voice in the Dark, The Face of Helen, The Dead Harlequin, The Bird With The Broken Wing, The Man From The Sea, Harlequin's Lane.
The book is dedicated by Christie "To Harlequin, The Invisible." No other Christie book is dedicated to a character. The author considered this character to be her favorite, and observes in 'Agatha Christie: An Autobiography' that Mr. Quin was a carryover from her Harlequin and Columbine poems, contained in The Road Of Dreams, and that both he and "little Mr. Satterthwaite" became favourite characters.