"Go on," Yale said with half-stifled impatience. "What's all this about trees?"

"The stumps," I said. "The tree men sawed the trees off close to the ground but left the stumps, and no one could get them out. A tractor came from a nearby farm and tried… " We'd had a great time then, having rides all day. "Anyway, it failed. Nothing else would move the stumps, and Fred didn't want to leave them there to rot, so he decided to blow them up… with black powder."

"Ah," Yale said.

Black powder had sounded, somehow, as if it ought to belong to pirates. We'd been most impressed. Fred had got his powder and he'd dug a hole down below the stubborn roots of the first stump, and filled it and set off one enormous explosion. It was just as well he'd cleared us out of the field first because the blast had knocked Fred himself flat although he'd been about a hundred feet away. The first tree stump had come popping out of the ground looking like a cross between an elephant and an octopus, but Malcolm, who came running in great alarm to see what had happened, forbade Fred to blow up the others.

As I told the gist of this to Yale and Smith, the second reel of the film was already unrolling in my mind, and I stopped fairly abruptly when I realised what I was remembering.

"Fred," I said, "carried the box of black powder back to the tool shed and told us never to touch it. We were pretty foolish but not THAT crazy. We left it strictly alone. And there the box stayed until it got covered over with other junk and we didn't notice it or think of it any more…" I paused, then said, "Wouldn't any explosive be useless after all this time?"

"Dynamite wouldn't last much more than a year in a tool shed," Smith said. "One hot summer would ruin it. But black powder – cordite – is very stable, and twenty years is immaterial."

"What are we waiting for?" Yale said, and walked towards the tool shed which lay behind the garage on the near side of the kitchen garden.

The tool shed was a place I hadn't thought of looking into the day before: but even if I had, I doubted if I would have remembered the black powder. Its memory had been too deep.

"Where is this box?" Yale asked.

I looked at the contents of the tool shed in perplexity. I hadn't been in there for years, and in that time it had passed from Fred to Arthur. Fred had had an upturned orange box to sit on while he waited through heavy showers: Arthur had an old fireside chair. Fred had had a tray with a cracked mug and a box of sugar cubes and had come indoors to fetch his tea: Arthur had an electric kettle. Fred had tended old tools lovingly: Arthur had shiny new ones with paint still on the handles.

Beyond the tools and the chair, in the centre section of the spacious shed, were things like mowers, chainsaws and hedge clippers and, at the furthest shadowy end, the flotsam by-passed by time, like the stuff in the cellar, stood in forgotten untidy heaps.

It all looked un promisingly undisturbed, but Yale called up a pair of young policemen and told them to take everything out of the tool shed and lay each object separately on the ground. Smith went back to the rubble, but Yale and I watched the policemen and so did Arthur Bellbrook, who came hurrying across the moment he saw what was happening.

"What's going on?" he said suspiciously.

"When did you last clean out the tool shed?" Yale asked.

Arthur was put out and beginning to bridle.

"Just say," I said to him. "We just want to know."

"I've been meaning to," he said defensively. "That's Fred's old rubbish, all that at the back."

The superintendent nodded, and we all watched the outgoing procession of ancient, rusting, broken and neglected tat. Eventually one of the men came out with a dirty wooden box which I didn't recognise at first because it was smaller than I'd seen in my memory. He put it on the ground beside other things, and I said doubtfully, "I think that's it."

"Mr Smith," Yale called.

Mr Smith came. Yale pointed at the box, which was about the size of crates used for soft drink bottles, and Smith squatted beside it.

The lid was nailed shut. With an old chisel, Smith prised it open and peeled back the yellowish paper which was revealed. Inside the paper, half-filling the box, there was indeed black powder.

Smith smelled it and poked it around. "It's cordite, all right, and in good condition. But as it's here, it obviously hasn't been used. And anyway, there wouldn't have been anything like enough in this box to have caused that much damage to the house."

"Well," I said weakly, "it was only an idea."

"Nothing wrong with the idea," Smith said. He looked around at the growing collection of discards. "Did you find any detonators?"

He had everyone open every single packet and tin: a lot of rusty staples and nails saw daylight, and old padlocks without keys and rotting batteries, but nothing he could identify as a substance likely to set off an explosion.

"Inconclusive," he said, shrugging, and returned to his rubble.

Yale told Arthur to leave the cordite where it was and do what he liked with the rest, and Arthur began throwing the decaying rubbish into the skip.

I tried to apologise for all the waste of time, but the superintendent stopped me.

"When you saw the tree stump blown up, which of your brothers and sisters were there?"

I sighed, but it had to be faced. "Gervase, Ferdinand and I were always together at that time, but some of the older ones were there too. They used to come for weekends still after they were grown up. Vivien used to make them, so that Malcolm wouldn't cut them out. Alicia hated it. Anyway, I know Lucy was there, because she wrote a poem about roots shrieking blindly to the sky."

Yale looked sceptical.

"She's a poet," I said lamely. "Published."

"The roots poem was published?"

"Yes."

"All right, then. She was there. Who else?" "Someone was carrying Serena on his shoulders when we had to leave the field for the explosion. I think it must have been Thomas. He used to make her laugh."

"How old were you all at that time?" Yale asked.

"I don't know exactly." I thought back. Alicia had swept out not very long after. "Perhaps I was thirteen. Gervase is two years older, Ferdinand one year younger. Lucy would have been… urn… twenty-two, about, and Thomas nineteen. Serena must have been six, at that rate, and Donald… I don't know if he was there or not… he would have been twenty-four."

Yale thoughtfully pulled out his notebook and asked me to repeat the ages, starting with Donald.

"Donald twenty-four, Lucy twenty-two, Thomas nineteen, Gervase fifteen, myself thirteen, Ferdinand twelve, Serena six."

"Right," he said, putting a full-stop.

"But what does it matter, if the cordite is still here?" I said.

"They all saw the force of the explosion," he said. "They all saw it knock the gardener over from a hundred feet away, isn't that what you said?"

I looked at the shattered house and said forlornly, "None of them could have done it."

Yale put his notebook away. "You might be right," he said.

Smith again came over to join us. "You've given me an idea," he said to me. "You and your tree roots. Can you draw me a plan of where the rooms were, exactly, especially those upstairs?"

I said I thought so, and the three of us went into the garage out of the wind, where I laid a piece of paper on the bonnet of Moira's car and did my best.

"The sitting-room stretched all the way between the two thick walls, as you know," I said. "About thirty feet Above that…" I sketched, "there was my room, about eight feet wide, twelve deep, with a window on the short side looking out to the garden. Malcolm's bedroom came next, I suppose about fifteen feet wide and much deeper than mine. The passage outside bent round it… and then his bathroom, also looking out to the garden, with a sort of dressing-room at the back of it which also led out of the bedroom…" I drew it. "Malcolm's whole suite would have been about twenty-two feet wide facing the garden, by about seventeen or eighteen feet deep."