Harley Quin is a mystery. He appears and disappears unexpectedly and by strange tricks of light and reflections; making his clothes seem brightly multicoloured like the theatrical Harlequin.

Published in various magazines through the 1920s, the short stories were collected and published for the first time in The Mysterious Mr. Quin in 1930 and in two other short stories: The Love Detectives and The Harlequin Tea Set.

Harley Quin helps his old friend Mr Satterthwaite to solve crimes through carefully worded but apparently random observations and questions that provide insight into a problem. Mr Harley Quin is there to act as a catalyst, to reveal to Mr Satterthwaite what he already knows.

There are strong hints that he is a spirit rather than human. Mr. Satterthwaite is always happy to encounter his elusive friend because it is on those few occasions that he ceased to be an observer and becomes an actor in the theatre of life. In one story, The Bird with the Broken Wing, Mr. Quinn contacts Mr. Satterthwaite by ouji board - which doesn't seem to surprise the latter at all.

In her autobiography, Agatha Christie listed the Harley Quin stories as her favourites and describes Harley Quin as "a friend of lovers and connected with death". The collection The Mysterious Mr. Quin is dedicated by the author "To Harlequin the invisible" which makes it unique as no other Christie book is dedicated to a character.

The 1928 silent movie The Passing of Mr. Quinn, which starred Stewart Rome, was loosely based on the short story The Coming of Mr Quin and was co-adapted by the film's director Leslie S. Hiscott who went on to direct Alibi starring Austin Trevor as Hercule Poirot. The character called Mr Quin in the movie is essentially a completely different person.

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