Agatha Christie: Lucy Worsley on the Mystery Queen

Lucy: One person who read all about it was Agatha Christie.

Years later, she wrote, "There was a case once where children had been neglected and abused after they had been placed by the council on a farm.

One child did die, and there had been a feeling that a slightly delinquent boy might grow up full of the desire for revenge."

The O'Neill case gave Agatha the germ of an idea that she used for a 30-minute radio play called "Three Blind Mice."

Here's the notebook where she's working on the idea, and she's done the title as a pictogram.

Three, blind-- that's an eye crossed out-- and then a cute little mouse.

But she couldn't use that title when she expanded the radio play into a full length stage play.

There was already a play with that title.

She was stumped.

She just couldn't think of an idea for what to call it, until somebody came up with a stroke of genius.

It was to be called "The Mousetrap."

( applause ) On the surface, the play Agatha developed is a classic Christie whodunit.

A group of unconnected strangers arrive at a guest house.

They become trapped there by bad weather.

It's another closed circle.

But soon Agatha will weave the O'Neill tragedy into the drama.

The play begins in quite a light-hearted way, but Agatha is so good at light and shade, things soon turn dark.

There's been a murder.

A policeman arrives at the house to follow up a lead, and he tells everybody that the victim was a woman who years before had been responsible for the death of a little boy in her care.

Was she murdered in revenge?

Taking inspiration from the real life O'Neill case, Agatha imagines that the surviving brother could be the vengeful killer.