Had I been in the city as a child of six? Had I felt the earth leap and split, watched half the city go up in flames in the worst fire America had ever seen? The disappearance of the first dream forced me to consider the possibility that Holmes was right, for it seemed almost as if, by naming the demon, he had stolen its authority.

Later in the afternoon of our last full day at sea, another image came to me that confirmed Holmes' interpretation beyond a doubt. The day was warm and bright and, passing under the ship's white canvas sun awnings, I was suddenly visited by a vision of my mother, wearing men's trousers, a ridiculous wide-brimmed straw hat with an enormous orange silk flower, and a delicious, self-mocking grin. She was turning from an open fire with a cast-iron skillet in one hand, a large spoon in the other, the bright canvas of an Army tent behind her; for a moment it was as if a door had been thrown open, permitting me, along with that tantalising glimpse, all the sensations the room-dream held: a thud of heavy sound beneath the crisp noise of breaking glass, a sharp thrill of terror, the feel of arms wrapping around me, and over it all an angry red haze. Then the door slammed shut, and I stood motionless for a long time, until a child ran past and broke my reverie.

It was, I knew without question, real. For that brief glimpse of recovered memory, I could forgive Holmes any degree of meddling. I could even admit to him that he was right: I had been in San Francisco during the earthquake, a child of six.

Why, however, had I pushed away all memory of the event?

We came at last to my childhood home, the West's biggest, youngest city, which spread over the end of a peninsula between ocean and bay. Eighty years ago, a ship coming through the Golden Gate would have seen nothing but a handful of Indian shacks clustered around a crumbling mission. Then, in 1848, John Marshall picked up a gleaming lump of yellow metal from a creek near Sutter's Mill, and the world came pouring in.