“I do not know where my brother is, but those are the addresses of Damian's gallery and his home. Either of those might be a good place to start.”

I looked at him in surprise. “You've simply been carrying this around?”

“When I heard that you were not with my brother, I knew it would not be long before you came looking.”

I grinned and gave him a quick kiss on the cheek, then reversed my direction.

“What shall I do with your valise?” he called after me.

I waved a hand in the air and broke into a trot.

To my surprise, the gallery that sold Damian Adler's paintings was not some narrow and dingy upper-storey hole several streets “off” Regent Street, but a prosperous, glass-fronted shop a stone's throw from the Royal Academy. A bell dinged at my entrance. Voices came from behind a partition at the back and a sleek woman in her early forties poked her head around the wall, giving me a brief but penetrating once-over. I did not think that I impressed her overmuch, since I had not intended to enact a patron of the arts when I left Sussex. “I shall be with you momentarily,” she said in a French accent.

“I'm happy to look,” I told her. She went back to her conversation, which had to do with the delivery of a painting.

The gallery had two rooms. The first displayed paintings, and a few small bronze sculptures, that would have been considered dangerously avant-garde before the War but were now just comfortably modern. I recognised an Augustus John portrait, and two of the bronzes were Epsteins. It was the next room that held the more demanding forms: one canvas made up of paint masses so thick, it could have been the artist's palette board mounted on the wall; three twisted sheets of brass that might be horses' heads or women's torsos, but in either case appeared to be writhing in pain; a gigantic, wide-brimmed cocktail glass tipped to pour its greenish contents into a puddle on the floor.

I spotted the first of Damian's paintings immediately I came into the room. It was an enormously tall, narrow canvas, twelve feet by two, and appeared at first glance to have been sliced from a larger, more complete image: branches and leaves at the top, giving way to a length of marvellously realistic bark and, at the bottom, the clipped grass out of which the tree was growing.

The centre of the image was a confusion of colours and shapes: a hand outstretched, a leg and foot dangling above the grass, and most troubling, a piece of a man's face with a staring, dead-looking eye. With a shock, I realised that I was looking at a strip, as it were, of a larger image, showing a man hanging from a tree-but if the eye was dead, that tensely extended hand was definitely not.

In a lesser craftsman, I would have thought he had painted the eye badly; in a lesser mind, I might have assumed the artist did not know how a dead hand would hang. But this was Damian Adler, so I looked at the card on which the title had been typed:

Woden in the World Tree

If I remembered my Norse mythology, the god Woden-or Odin-had hanged himself for nine days in the tree that supported the world, so as to gain knowledge. Woden was blind in one eye.

I nodded in appreciation, and moved to the next painting, that of a hand shaking itself in a mirror-clever, but nothing more. The one after that appeared to be a solid wall of leaves, meticulously detailed, until one noticed that the twin points of gleam to one side were eyes: The hidden image gradually resolved into the ancient pagan figure of the Green Man.

Next time I walked through the woods, the back of my neck was going to crawl.

The room's far wall at first glance seemed to have a window in it, but did not.

What it had was a trompe-l'oeil painting, with shadows falling naturally both on the inner sill and on the scene “outside.” It showed an alleyway, such as might indeed be on the other side of the wall: an expanse of dirty red bricks topped by a slice of sky. At the upper edge of the canvas was a crescent moon, translucent in the bright daylight. A man strode towards the right-hand edge of the canvas, his hat tipped back on his head, his right hand swinging forward, grasping some object that was cut off by the edge of the canvas-although something about his posture made one think he was perhaps being pulled along by whatever was in his hand.

The painting reminded me of something: I walked forward to see if I could figure out what.

Up close, everything changed. The bricks began to glisten and take on the texture of living matter, as if skin had been flayed from a muscle wall. Closer still, the cracks and mortar grew alive with tiny creatures, squirming and baring sharp, minuscule teeth; the pale shape in the upper corner suggested less a daylit moon than it did a mouth, poised to open. Taking a step back felt like a natural response.

I was not surprised to see the signature in the corner: The Addler. Suddenly, I saw why it looked familiar: If the brick walls had been sandbags and the businessman replaced by three soldiers, I would be looking at his 1915 drawing of the trench under fire.

“Mesmerising, is it not?” came a French accent from behind me.

“Disturbing,” I said.

“Great art often is.”

I thought about that. Was it possible that time would declare Holmes' son great? That the peculiarity of Damian's work was less the sign of a troubled mind than the fearless exploration of an artistic vision? Many had thought Holmes himself unbalanced. “Great or not, I don't know that I'd want it in my sitting room.”

It was the wrong thing to say: When I turned, the woman had raised a polite and condescending face. “Surrealism expresses thought without reason, pure artistic impulse with no hindrance from rationality or aesthetics. Perhaps you should take a closer look at the other room. Vanessa Bell has just sent me a very nice portrait that would look good on a sitting room wall.”

I hastened to get back into the woman's better graces. “Oh no, I like Damian's work enormously. I like him, for that matter. It's just that some of his paintings are, what? A little too compelling for comfort?”

The small woman tipped her perfect head at me, considering. She herself was an artifice-at any rate, a flawless appearance and a sympathy for Bohemian artists did not go hand-in-hand. In the end, she decided that I, too, was not what I appeared.

“You have met Mr Adler?”

“I've known him for years,” I said, which was the literal, if not the complete, truth. “He came to dinner the other night. When I heard you were displaying his work, I thought I'd stop in. This is another of his, isn't it?”

The other painting, on the room's back wall, bore his characteristic hand: painful, nightmare images painted with such loving realism, one was tempted to reach out and touch the surface, just to reassure one's self that it was two dimensional.

The moon, again. Only this time, it was a pair of moons, two bright eyes in the night-time sky, staring down at the eerie blue-tinged outlines below. The shapes of the landscape were difficult to determine. At first I thought it was a group of bulky figures walking along an unlit street. Moving closer, I noticed that the shapes were nearly square: tall buildings in a modern city during an electrical outage? The painting occupied the room's darkest corner, which did not help any. But when I was nearly on top of it, the details became clear.

The painting showed a prehistoric site, a grouping of massive stones both upright and fallen, forming a rough circle on a moonlit hillside. The grass around them was composed of a million delicate black and blue-black brushstrokes, the texture of a cat's fur.

I lifted my gaze to the dual moons, and saw that the craters and patterns on their near-white surfaces had been re-arranged to suggest a retina and iris: Two great pale eyes gazed down from a sable sky.