Dom entered. His grandmother glanced up, and nodded towards a chair. The air was musty with incense.

The large white-painted room was completely empty except for the small desk and two chairs and the little standard thurible and altar in one corner, though Joan had a way of filling up empty spaces with her presence.

In foot-high letters along the facing wall the ubiquitous One Commandment glared down on them.

Joan closed her account book and began to play with a white-hi lt ed knife.

'In a few days it'll be Soul Cake Friday, and also the Eve of Small Gods,' she said. 'Have you given much thought to joining a klatch?'

'Not much,' said Dom, who hadn't thought at all about his religious future.

'Scares you, eh?'

'Since you put it like that, yes,' said Dom. 'It's a rather final choice. Sometimes I'm not sure Sadhimism has all the answers, you see. '

'You're right, of course. But it does ask the right questions.' She paused for an instant, as if listening to a voice that Dom could not hear.

'Is it necessary?' prompted Dom.

'The klatch? No. But a bit of ritual never did anyone any harm, and of course it is expected of you.'

'There is one thing I'd like to get clear,' said Dom.

'Go ahead.'

'Grandmother, why are you so nervous?'

She laid down the knife and sighed.

'There are times, Dom, when you raise in me the overwhelming desire to bust you one on the snoot. Of course I'm nervous. What do you expect?' She sat back. 'Well, shall I explain, or will you ask questions?'

'I'd like to know the story. I think I've got some kind of right. A lot has been happening to me lately, and I kind of get the impression that everyone knows all about it except me.'

Joan stood up, and walked over to the altar. She hoisted herself on to it and sat swinging her legs in an oddly girlish way.

'Your father - my son - was one of the two best probability mathematicians the galaxy has ever seen. You have found out about probability maths, I gather. It's been around for about five hundred years. John refined it. He postulated the Pothole Effect, and when that was proved, p-math went from a toy to a tool. We could take a minute section of the continuum - a human being, for example - and predict its future in this universe.

'John did this for you. You were the first person ever quantified in this way. It took him seven months, and how we wish we knew how he managed it, because even the Bank can't quantify a person in less than a year with any degree of accuracy. Your father had genius, at least when it came to p-math. He ... wasn't quite so good at human relationships, though.

She shot an interrogative glance at Dom, but he did not rise to the bait. She went on: 'He was killed in the marshes, you know.'

'I know.'

John Sabalos looked out over the sparkling marshes, towards the distant tower. It was a fine day. He surveyed his emotions analytically, and realized he felt content. He smiled to himself, and drew another memory cube towards him and slotted it in the recorder.

'And therefore,' he said, 'I will make this final prediction concerning my future son. He will die on his half-year birthday, as the long year is measured on Widdershins, which will be the day he is invested as Planetary Chairman. The means: some form of energy discharge.'

He switched off for a few seconds while he collected his thoughts, and then began: 'The assassin: I cannot tell. Don't think I haven't tried to find out. All I can see is a gap in the flow of the equations, a gap, maybe, in the shape of a man. If so, he is a man around whom the continuum flows like water round a rock. I know that he will escape. I can sense him outlined by your actions like - damn, another simile - a vacuum made of shadow. I think he works for the Joker Institute, and they are making a desperate attempt to kill my son.'

He paused, and glanced down at his equation. It was polished, perfect, like a slab of agate. It had an intrinsic beauty.

The distant glint of the Tower drew his gaze again. He glanced up. Not the right time, not yet. Another hour ...

'And now, Dom, as you stand there torn between shock and astonishment, what do you see? Does your grandmother have that tight-lipped, determined look she wears at times of stress? How was the party, anyway?

'Dom, you are my son, but as you are perhaps learning, I have many sons - untold millions. Have, I say, but "had" I mean. For in those billions of universes that hedge us about on every side, they are dead as I predicted. You, who are flesh and blood, are also that one chance that lies a long trek behind the decimal point. That chance that I am wrong. But a student of probability soon realizes that by its nature the billion-to-one chance crops up nine times out of ten, and that the greatest odds boil down to a double-sided statement: it will happen, or it will not.

'I have studied you, and the billion-to-one universe in which you now stand. It left the main-sequence universe at the point of your non-death. Universes are like the stars which some of them contain. Most follow the well-beaten path. But some, by the twist of a photon, career down strange histories which end in super-novae or impossible holes in space. Rogue universes now, crack under the stress of paradox or - what?

'I will try to give you some help, because you will need it. Your assassin came from your present universe, can you understand that? He wanted to prevent you discovering something that will make your chance-in-a-billion universe the greatest in all the alternate creations. But I've an inkling that whatever saved you from death came from your universe, too. I've seen a lot in your universe but how can I tell you because, believe me, Dom, if I did the paradox burden would split your universe at the seams.'

He laid down the recorder and wandered idly into his outer office. The secretary robot clicked into life.

'If anyone calls I am going out to the Tower. I, uh, shouldn't be long.'

'Yes, Mr Chairman.'

'You'll find a cube on my desk. Please send it to Her Managing Directorship.'

'Certainly.'

John Sabalos closed the door and went back to his desk. He was still wearing his black and brown robes from the Hogswatch celebrations of the night before. He hadn't slept, but he felt exhilarated. It was false, of course. Knowing the future wasn't the same thing as controlling it. It just felt like it. He picked up the recorder.

'This I can say, however. Three things. You will discover the Jokers World, if you look in the right directions. Your life will be in danger. And, thirdly... look up in the corner of the room! Run for your life!'

He switched off, and laid the cube on his desk.

Somewhere outside, over towards the east lawn, someone was playing the phnobic chlong zither, badly. John stepped outside. The clatter of Joan's old electric computer floated up from the kitchen domes, which meant she was processing the eighth-year household accounts.

He breathed deeply. Something was adding a third dimension to his senses, etching the external world in high relief. With a probability adept's skill he located the cause. The world was like wine, because this was his last day in the world. The last of the wine. And, they would kill him before he discovered Joker's World. Dom should be luckier.

His personal flyer bobbed in the swell, down by the long jetty.

The door slid to. With a light tread, he set off, quelling the wild elation that ran through him, because death was a serious matter.

His father's voice stopped and the cube projection stopped. Dom shot a glance upwards.

Something small glittered in the air, like a mote of metallic dust. He heard Joan's voice, every word as crisp as frosty air.