On a winter's night in 1974, a crime took place that would obsess the nation.

It was an extraordinary news story.

The story of an aristocrat, Lord Lucan, who's said to have killed the family Nanny,

mistaking her for his wife, then somehow just disappeared.

One of the great mysteries in English criminal history. We're still looking for Lucan.

It's honestly one of the most powerful stories of my lifetime.

I'm Alex Fontunzelman.

This is The Lucan Obsession.

Listen on BBC Sounds.

BBC Sounds, music, radio, podcasts.

Hello, today we're getting involved in one of the fundamental units of narrative drama,

The Family, from those torn apart by secrets to others brought together through displacement

and another united, not by blood, but by a world-famous and famously mysterious Polish novelist.

We start, though, with the moment when one North

London family implodes.

I don't know if these things are such a good idea, Tom grumbled, reading the small print on his

DNA test. They collect your data. You're putting your genetic material in the hands of

organisations you know nothing about. Don't take the test then, Lena said, sulky.

I don't think any of you should take it. The urgency in his voice silenced the room. He looked up at

them all. His face looked as though it was collapsing. Please don't take the test. Lina felt

something shift inside her. Her father famously never said please and thank you. Why shouldn't we take it? Allison asked. Because I said so, he said, the way he had when they were little. Just don't, okay? But why? Lina was beginning to panic. What's going on, Dad? He couldn't look at them. Something was very wrong. You're dying, she said,

just to get the words out to get it over with. But he shook his head. Your mother said this would happen.

...