Mysterious Mr Quin (1930)

Plot: We meet a supernatural love detective

Christie takes a sharp left here. This is clearly the same author who created Hercule Poirot, but this is also the same author behind the mystical bittersweet collection of While The Light Lasts.

Mr Quin and Mr Satterthwaite are a fascinating duo. Mr Satterthwaite is a well-meaning man of means who has never really taken part in life. He lives in luxury, but is entering his retirement, and cannot see himself ever loving, so contents himself with sharply observing others. He's fashionable and snobbish and fussy - but he's also concerned about the happiness of other people, and takes a keen interest in human nature.

In many ways, he's an equivalent of Poirot - a man content to travel the world, occasionally becoming involved in adventurers, watching everything with his beady eye. But he isn't brilliant - his brain needs that extra push.

The extra push comes from Mr Harley Quin, a mystical figure who comes and goes like magic, part of the ancient Harleyquinade (a pantomime that turns up several times in Christie). Quin appears like a benevolent, sometimes avenging, spirit of love.

It is Quin who uncovers long-ago crimes, allowing suspicious lovers to realise that both are innocent. It is Quin who confronts the Croupier and fallen, fashionable ruin of his first wife, and allows them to forgive each other. Quin is a malevolent spirit - and his meetings with Satterthwaite are nothing but ordained. Unlike those Poirot cases where the reader's eyes roll up at the sheer coincidence, Mr Quin is clearly a supernatural power restoring order to the world, summoning Satterthwaite to help him.

It is the last story that the ambiguous nature of Quin is teased at with the Lovers' Lane, where Mr Satterthwaite gets a glimpse of "something at once menacing and terrifying... Joy, Sorrow, Despair."

At the end, we have Satterthwaite alone, humbled before ...what?, forced to ask himself whether his contented life without love has been worth it after all.