When I started to follow Scilla into the First-Aid room the doctor put his hand on my arm to stop me.

'You're Mr York, aren't you?' he asked. He had given me a regulation check after an easy fall I'd had the day before.

'Yes.'

'Do you know these people well?'

'Yes. I live with them most of the time.'

The doctor closed his lips, tight, thinking. Then he said, 'It's not good. The concussion's not much, but he's bleeding internally, possibly from a ruptured spleen. I've telephoned the hospital to take him in as an emergency case as soon as we can get him there.'

As he spoke, one of the racecourse ambulances backed up towards us. The men jumped out, opened the rear doors, took out a big stretcher and carried it into the First-Aid room. The doctor went in after them. Soon they all reappeared with Bill on the stretcher. Scilla followed, the anxiety plain on her face, deep and well-founded.

Bill's firm brown humorous face now lolled flaccid, bluish-white, and covered with fine beads of sweat. He was gasping slightly through his open mouth, and his hands were restlessly pulling at the blanket which covered him. He was still wearing his green and red checked racing colours, the most ominous sign of all.

Scilla said to me, 'I'm going with him in the ambulance. Can you come?'

'I've a ride in the last race,' I said. 'I'll come along to the hospital straight after that. Don't worry, he'll be all right.' But I didn't believe it, and nor did she.

After they had gone I walked along beside the weighing room building and down through the car park until I came to the bank of the river. Swollen from the recently melted snow, the Thames was flowing fast, sandy brown and grey with froths of white. The water swirled out of a mist a hundred yards to my right, churned round the bend where I stood and disappeared again into the fog. Troubled, confused, not seeing a clear course ahead. Just like me.

For there was something wrong about Bill's accident.

Back in Bulawayo where I got my schooling, the mathematics master spent hours (too many, I thought in my youth) teaching us to draw correct inferences from a few known facts. But deduction was his hobby as well as his job, and occasionally we had been able to side-track him from problems of geometry or algebra to those of Sherlock Holmes. He produced class after class of boys keenly observant of well-worn toe-caps on charwomen and vicars and calluses on the finger tips of harpists; and the mathematics standard of the school was exceptionally high.

Now, thousands of miles and seven years away from the sunbaked schoolroom, standing in an English fog and growing very cold, I remembered my master and took out my facts, and had a look at them.

Known facts- Admiral, a superb jumper, had fallen abruptly in full flight for no apparent reason. The racecourse attendant had walked across the course behind the fence as Bill and I rode towards it, but this was not at all unusual. And as I had cleared the fence, and while I was looking down at Bill, somewhere on the edge of my vision there had been a dull damp gleam from something grey and metallic. I thought about these things for a long time.

The inference was there all right, but unbelievable. I had to find out if it was the correct one.

I went back into the weighing room to collect my kit and weigh out for the last race, but as I packed the flat lead pieces into my weight cloth to bring my weight up to that set by the handicapper, the loudspeakers were turned on and it was announced that owing to the thickening fog the last race had been abandoned.

There was a rush then in the changing room and the tea and fruit-cake disappeared at a quickened tempo. It was a long time since breakfast, and I stuffed a couple of beef sandwiches into my mouth while I changed. I arranged with Clem for my kit to go to Plumpton, where I was due to ride four days later, and set off on an uninviting walk. I wanted to have a close look at the place where Bill had fallen.

It is a long way on foot from the stands to the far end of Maidenhead racecourse, and by the time I got there my shoes, socks, and trouser legs were wet through from the long sodden grass. It was very cold, very foggy. There was no one about.

I reached the fence, the harmless, softish, easy-to-jump fence, made of black birch twigs standing upright. Three feet thick at the bottom slanting to half that size at the top, four feet six inches tall, about ten yards wide. Ordinary, easy.

I looked carefully along the landing side of the fence. There was nothing unusual. Round I went to the takeoff side. Nothing. I poked around the wing which guides the horses into the fence, the one on the inside of the course, the side Bill had been when he fell. Still nothing.

It was down underneath the wing on the outside of the course that I found what I was looking for. There it lay in the long grass, half hidden, beaded with drops of mist, coiled and deadly.

Wire.

There was a good deal of it, a pale silver grey, wound into a ring about a foot across, and weighted down with a piece of wood. One end of it led up the main side post of the wing and was fastened round it two feet above the level of the top of the birch. Fastened, I saw, very securely indeed. I could not untwist it with my fingers.

I went back to the inside wing and had a look at the post. Two feet above the fence there was a groove in the wood. This post had once been painted white, and the mark showed clearly.

It was clear to me that only one person could have fixed the wire in place. The attendant. The man whom I myself had seen walk across from one side of the course to the other. The man, I thought bitterly, whom I had left to help Bill.

In a three mile 'chase at Maidenhead one rode twice round the course. On the first circuit there had been no trouble at this fence. Nine horses had jumped it safely, with Admiral lying third and biding his time, and me riding alongside telling Bill I didn't think much of the English climate.

Second time round, Admiral was lengths out in front. As soon as the attendant had seen him land over the fence before this one, he must have walked over holding the free end of the wire and wound it round the opposite post so that it stretched there taut in the air, almost invisible, two feet above the birch. At that height it would catch the high-leaping Admiral straight across the shoulders.

The callousness of it awoke a slow deep anger which, though I did not then know it, was to remain with me as a spur for many weeks to come.

Whether the horse had snapped the wire when he hit it, or pulled it off the post, I could not be sure. But as I could find no separate pieces, and the coil by the outer wing was all one length, I thought it likely that the falling horse had jerked the less secure end down with him. None of the seven horses following me had been brought down. Like me, they must have jumped clear over the remains of the trap.

Unless the attendant was a lunatic, which could by no means be ruled out, it was a deliberate attack on a particular horse and rider. Bill on Admiral had normally reached the front by this stage in a race, often having opened up a lead of twenty lengths, and his red and green colours, even on a misty day, were easy to see.

At this point, greatly disturbed, I began to walk back. It was already growing dark. I had been longer at the fence than I had realized, and when I at length reached the weighing room, intending to tell the Clerk of the Course about the wire, I found everyone except the caretaker had gone.

The caretaker, who was old and bad-tempered, and incessantly sucking his teeth, told me he did not know where the Clerk of the Course could be found. He said the racecourse manager had driven off towards the town five minutes earlier. He did not know where the manager had been going, nor when he would be back; and with a grumbling tale that he had five separate stoves besides the central boiler to see to, and that the fog was bad for his bronchitis, the caretaker shuffled purposefully off towards the dim murky bulk of the grandstand.