We set off at a trot, each of us clutching a spear with a tip of sharp flint, and a thick wooden club with a fist-sized rock securely tied on one end with strips of leather.

We get to the end of the beach – exactly like the real beach where we live in Tynemouth (apart from the mammoths, obviously) – and run up the hill until we’re staring out over the huge plain where, in maybe ten thousand years’ time, there will be a wide road, and a pub playing live music, and a housing estate of low-rise flats. Now there isn’t any of that. There isn’t anything made by humans – apart from an old-fashioned airship that’s floating past in the sky above, shaped like a giant goldfish. Don’t ask me what it’s doing there. Dreams are weird like that and, by now, I’m kind of used to it.

There is no sign of our friends, though.

I say, ‘Super-sprint. Dream-style. You up for it?’

Seb grins gappily, and in an instant we are sprinting across the windy plain like a pair of Olympic runners battling for the finish line. Side by side, weapons in hand, I’m edging ahead of Seb, and then he pulls level as the Gravy Lake comes into view in the far distance. Then he’s ahead of me. He remains ahead as we descend the side of the shallow canyon where there is a green river of minty custard (this is a dream, remember?) and we hop across the exposed rocks and up the other side.

I let him get a good lead so that he will think he is winning. Then it’ll be an easy matter to lengthen my stride, judging it finely so that I can overtake him and win at the last minute, but not humiliate him so that he won’t want to race again.

And so, as the Gravy Lake gets closer, and I can see the shapes of our companions gathering on its shore, I begin to exert myself a little more. I deliberately make my strides stronger and longer … but still Seb is ahead of me. I drop my weapons and pump my arms more, thrusting my chin out, and run harder. And harder.

It’s happening again. My dream is not doing what I tell it to do.

What’s wrong? I’m not gaining on Seb at all.

I have no idea exactly how fast we are running, but the ground is whooshing past under my feet at a terrifying rate and, however fast I go, Seb is managing to keep ahead of me.

It is not meant to happen like this. I don’t understand it.

Kobi and the others are in full view now, and I can’t stop in time. I’m going so fast that I run right past them and into the shallows of the freezing-cold lake where the watery school gravy finally stops me and I fall forward, sinking under the surface before rising, gasping for air. The others point at me and laugh, while Seb bounces on his feet, arms raised in victory.

The cold of the gravy has shocked me.

Being beaten by Seb has shocked me more.

I’m still standing in the shallows of the brown lake, and I look round at the group: there’s little Erin, old Farook and, of course, Kobi the Cave Boy who looks like he does in Seb’s book, which is cartoonish. He’s basically a walking, talking drawing. He is wearing a fur thing that only covers one shoulder and he has a club-and-rock weapon like the one I just ditched. Looking at his fur makes me feel even colder because I’m just wearing my soaking pyjamas. I close my eyes and say, ‘Change pyjamas to fur,’ and wait.

Nothing happens. I try again, but I’m already losing confidence.

Seb hasn’t seen any of this: he’s a few metres away, talking to the others. I call over to him and he saunters back, all cocky after beating me in a running race.

‘What’s up, loser?’ he says. ‘You not cold?’

‘Seb,’ I say, ‘it’s going wrong again.’

‘What do you mean, “again”?’ says Seb.

‘I’ve told you: the dream doesn’t always do as it’s told, and it’s happening much quicker now. Look!’ I point upwards. ‘Turn green!’ The sky does not turn green. I don’t want to scare him, though. Instead, I say, ‘Shall we wake up now?’ It’s really the only safe option.

He wrinkles his nose and pouts. ‘I don’t want to. What’s wrong with you? You said it yourself, Malky. We haven’t got much time. I want to get to the bit when I ride the mammuf, at least!’

He’s in such a good mood, and he’s probably right. Even if I can’t direct things perfectly, we’ll both come out of the dream cycle, anyway, waking up normally in our beds at home, in about twenty minutes. I’ll soon dry out.

Relax, Malky! It’ll be perfectly safe. Just like a normal dream where weird stuff happens.

I try to convince myself, I really do. I tell myself, Let it be …

‘Come on, Malky,’ he says. ‘We’re on a food raid, remember? Just like in the book!’

‘Yeah, yeah,’ I sigh. ‘You win.’

I move forward on to the lip of the low cliff, where the lake tumbles over the rock in a massive waterfall, like the drawing in the book. I release the big breath that I took and sniff the air, turning my head completely in line with the horizon.

The smell is coming from where the sun is just beginning to set, painting the Gravy Lake brownish-pink. Someone is roasting meat. Mammoth? I turn back to the others and nod. ‘Meat,’ I say. Kobi’s cartoon lips part in a wide grin and he sticks out his tongue with pleasure. He has no fear about what might come next. He never does. Beside him, Erin stands up and holds her hand out to old Farook who waves it away and gets to her feet with a small grunt.

(Seb made up most of the names, by the way. Just thought I’d say that. Erin is a kid in his class.)

Through the trees, there’s a huge rock and, a little further on, the faint glow of a fire.

Stealing meat from another tribe is a huge risk. In the book, it’s all fine and happy-endy: the tribe gives us meat because we’re hungry, then Kobi gets to ride on a mammoth. We’ve never actually got that far in the dream, we’ve always been side-tracked. It’s probably why Seb doesn’t want to leave. He really wants to ride that mammoth, and I can’t say I blame him.

I crouch behind the rock and pick up a lump of dirt and sniff it, recoiling at the foul smell of dog poo. ‘Dogs,’ I whisper, wiping it off my fingers. Even in the dark, I see a flash of fear pass over Erin’s face. We all know about the dogs. The other tribe travels with them. They can talk to them, give them names and commands, just like we do in real life. The dogs attack when told to. They are terrifying, even in a dream.

Then from behind me I hear a sound: r-r-r-r-r-r-r-r-r-r-r-r-r-r. I swallow and spin round: there it is. An old black-and-ginger hound with a grey muzzle. Its head is held low, ready to pounce; its eyes flash amber in the low sun. It lifts up one misshapen front paw, twisted from some old injury, and growls again.

R-r-r-r-r-r-r. There is another one now, and another. We turn … but they’re behind us too. The five of us – me, Seb, Kobi, Erin and Farook – are blocked from retreating.

Trapped.