It’s important that you understand two things about Agatha Christie.

The first thing you have to understand is that Agatha Christie was fiercely sane. She spent her entire life being pragmatic, and methodical; except for the time she went mad.

She was born in 1890, in the English seaside town of Torquay, into an idyllic world. Her parents had what she described as “that rare production: a happy marriage.” On the eve of the First World War, she agreed to marry her golfing partner Reggie Lucy, whom she thought seemed a sensible partner. Then she met Archie Christie at a dance, and everything changed.

Her relationship with Archie was tempestuous. They got engaged, broke it off, got back together, fought. She was miserable. She was in love. The first wind of the madness to come, winging its way towards her. “I had been so happy, so peaceful with Reggie, we had understood each other so well; we’d enjoyed and wanted the same things. What happened to me now was the opposite. I loved a stranger; mainly because he was a stranger.”

Then, the Great War came. Archie and Agatha married, and Archie was mobilised. Agatha went to work in the war hospitals as a nurse. In the wards, she watched as men died. Everywhere, senseless death. And at home, she wrote her first mystery. The Mysterious Affair at Styles.

It was rejected by publisher after publisher. Agatha forgot about it, in the endless grind of wartime England. She didn’t think she was a writer, anyway; it had been a bet with her sister, a distraction. Archie came back, the war ended, they had a child, Rosalind, and then, suddenly, after two years, a publishing house picked up the manuscript. Agatha signed the contract without even looking at it, despite the fact that it bound her to the house for another five books, and hardly paid her a penny. It didn’t matter. She wasn’t going to be a ‘real’ writer anyway.

That night, when she and Archie went out to celebrate at the Hammersmith Palais de Danse, “there was a third party with us, though I did not know it. Hercule Poirot, my Belgian invention, was hanging round my neck, firmly attached there like the old man of the sea.”

Everything seemed to be going so well. She wrote six more books, travelled the world with Archie, and together they bought a house, which he insisted on naming ‘Styles’. Slowly, her popularity as a writer was growing. And then, in 1926, disaster struck. Her mother died.

Packing up her childhood home, emptying rooms and cupboards, struck down with grief, Agatha turned inwards. Archie tried to persuade her away with him, on a trip to Spain, but she refused. She knew that we all end up orphans; she didn’t want to be distracted from her sorrow. She wanted to sit with it, get used to it. Agatha always thought her first, her most important job, was to be a wife. And later, she would think she’d made a mistake. “I see now that I was wrong. My life with Archie lay ahead of me. We were happy together… but he hated the feeling of sorrow in the house, and it left him open to other influences.”

The other influence was a dark-haired secretary in London. When Archie told her that he wanted a divorce, just months after her mother’s death, Agatha fell apart. They tried to patch it up but it only made things worse; he became more ruthless, she more retiring.

Then she disappeared.

Her abandoned car was found near a local lake, the Silent Pool, where she’d killed off one of her characters. The tabloids exploded. There was a national manhunt. Archie was accused of killing her; his phone was tapped. Men combed the nearby countryside, 300 of them, five yards apart, stalking over ten miles of ground. Other detective writers put out theories for the papers. Even Arthur Conan Doyle got involved, taking one of her discarded gloves to a psychic for guidance.

In her 542-page autobiography, Agatha makes no mention of the eleven days she was missing. In place of the explanation she would never give, she quotes Keats. What shall I do to drive away / Remembrance from mine eyes. And then she asks, “But should one drive it away? If one chooses to look back over the journey that has been one’s life, is one entitled to ignore those memories that one dislikes? Or is that cowardice? I think, perhaps, one should take one brief look, and say: ‘Yes, this is part of my life; but it’s done with. It is a strand in the tapestry of my existence. I must recognise it because it is part of me. But there is no need to dwell upon it.’

And that is all.

She was discovered in a health hotel in Yorkshire, under an assumed name, with apparently no memory of who she was, or how she came to be there. Archie told the press she’d had a brief lapse into amnesia. At the time, no one believed her. It was a publicity stunt. A way to punish her husband. A ruse. A trick.

Psychologists have since called it a fugue state, her collapse. A musical fugue has more than one voice. Each voice takes the theme, spins it, over and over, a building of sliding repetitions until the single theme is lost in something new. Maybe that was what it was like for her, watching the person she had wanted to be—the happy marriage, the reasonable world—slip away behind her.

They divorced, and Agatha left England. To travel alone for the first time in her life. “To find out what sort of person I was.”

She was a phenomenon, it turned out. That period was when she felt she changed from an amateur into a professional writer. In Iraq, she met the young archaeologist Max Malomar, fifteen years her junior, who she was to marry and stay with until her death forty-five years later. Together, they travelled the world conducting digs, and she wrote to fund their expeditions. She went on to pen seventeen plays, 150 short stories, and sixty-six novels, which have sold over two billion copies. After God and Shakespeare, Agatha Christie is the third highest selling author in the world. And those two had a head start on her, anyway.

At seventy-five, when Agatha finished the fifteen-year process of writing her autobiography, she chose to end it like she had lived, through two world wars, two marriages, and across continents, determined to make sense of a chaotic world. Making no excuses.

“I have remembered, I suppose, what I wanted to remember; many ridiculous things for no reason that makes sense. That is the way we human creatures are made. And now that I am seventy-five, it seems the right moment to stop. Because, as far as life is concerned, that is all there is to say. I live now on borrowed time, waiting in an ante-room for the summons that will inevitably come. And then—I go on to the next thing, whatever it is. One luckily doesn’t have to bother about that.” She lived for another ten years, and died, still working.

The second thing you have to understand about Agatha Christie is that there is a reason behind every unhappiness, every death, between her covers. Not like the confusing, violent place outside. Inside Agatha’s world, there is good and evil, and good will triumph, evil be punished. Every death will be chased down, not melodramatically, but matter-of-factly, methodically, because that is how it’s done. Every life accounted for. Every mystery solved.