A shopping trolley rolled at high speed up the aisle, with shoppers running to get out of the way. It had a plastic window frame hanging on the front and was splashed with artificial potato. Johnny and Kirsty were hanging on either side.

He waved at them as he drifted past.

"Help us get this out of the back door!"

"That's old Mrs Tachyon's trolley, isn't it?" said Yoless.

"Who cares?" said Bigmac. He put his burger down on the edge of the pond, where it was surreptitiously picked up by Wobbler, and ran after the trolley.

"Someone's chasing us," Johnny panted, as they caught up.

"Brilliant!" said Bigmac. "Who?"

"Some people in a big black car," said Johnny. "Only ... they've vanished ... "

"Oh, an invisible big black car," said Yoless.

"I see them all the time," said Bigmac.

"Are you going to stand around all day?" Kirsty demanded. "It's probably got some kind of special shield! Come on!"

The trolley wasn't massively heavy, although the piles of bags did weigh it down. But it did need a lot of steering. Even with all of them helping - or, Johnny thought later, perhaps because of all of them helping it skidded and wobbled as they tried to keep it in a straight line.

"If we can get out of the other doors, we're in the High Street," said Johnny. "And it can't go in there because there's bollards and things."

"I wish I had my five-megawatt laser cannon," said Bigmac, as they fought the trolley round a corner.

"You haven't got a laser cannon," said Yoless.

"I know, that's why I wish I had one."

Ow.

Wobbler leapt back.

"It bit me!" he screamed.

Guilty stuck his head out of the heap of bags and hissed at Johnny.

Security guards were strolling towards them. There were five kids arguing around a trolley, Bigmac was among them and, as Yoless would have pointed out, one of them was black. This sort of thing attracts attention.

"This trolley might be a time machine," said Johnny. "And that car ... Kirsty thinks someone's after it. I mean me. I mean us."

"Great, how do we make it work?" said Bigmac.

"A time machine," said Yoless. "Ali. Yes?"

"Where's this invisible car got to?" said Wobbler.

"We can't go out of the other doors," said Kirsty, flatly. "There's a couple of guards there."

Johnny stared at the black dustbin liners. Then he picked one up and undid the string. For a moment his fingers felt cold and the air was full of faint whispers

The mall vanished.

It vanished above them, and around them.

And below them.

They landed in a heap on the grass, about a metre below where they'd been standing. The trolley landed on top of them, one wheel slamming into the small of Johnny's back. Bags bounced out, and Guilty took the opportunity to scratch Bigmac's ear.

And then there was silence, except for Bigmac swearing.

Johnny opened his eyes. The ground sloped up all around him. There were low bushes at the tap.

"If I asked what happened," said Yoless, from somewhere under Bigmac, "what'd you say?"

"I think we may have travelled in time," said Johnny.

D'you get an electric feeling?" said Wobbler, clutching his jaw. "Like ... all your teeth standing on end?"

"Which way did we go?" said Yoless, still talking in his deliberate voice. "Are we talking dinosaurs, or mutant robots? I want to know this before I open my eyes.

Kirsty groaned.

"Oh dear, it's going to be that kind of adventure after all," she hissed, sitting up. "It's just the sort of thing I didn't want to happen. Me, and four token boys. Oh, dear. Oh, dear. It's only a mercy we haven't got a dog." She sat up and brushed some grass off her coat. "Anyone got the least idea of where we are?"

"Ali," said Yoless. "I see there's grass. That means no dinosaurs. I saw that in a film. Grass didn't evolve until after there were dinosaurs."

Johnny stood up. His head was aching. He walked to the edge of the little hollow they'd landed in, and looked out.

"Really. Someone's been paying attention," said Kirsty. "Well, that narrows it down to some time in the last sixty million years."

"Proper time travellers have proper digital readouts," Wobbler grumbled. "No grass? What did dinosaurs eat, then?"

"You only get digital time machine clocks in America," said Bigmac. "I saw a film about a time machine in Victorian England and it just had light bulbs. They ate other dinosaurs, didn't they?"

"You're not allowed to call them dinosaurs any more, " said Yoless. "It's speciesist. You have to call them pre-petroleum persons."

"Yeah," said Bigmac. "One Million Years PC. Get it? "Cos there was this film called One Million Years BC, but-"

Kristy's mouth was open.

"Do you lot go on like this all the time?" she said. "Yes, you do. I've noticed it before, actually. Rather than face up to facts, you start yakking on about weird things. When are we?"

"May the twenty-first," said Johnny, sitting down next to her. "Just gone half past three."

"Oh yes?" said Kirsty. "And how come you're so sure?"

"I went and asked a man who was walking his dog," said Johnny.

"Did he say what year?"

Johnny met Kristy's gaze. "No," he said. "But I know what year."

They climbed out of the hollow and pushed their way through the bushes.

A scrubby field stretched away below them. There were some allotment gardens at the bottom end of the field, and then a river, and then the town of Blackbury.

It was definitely Blackbury. There was the familiar rubber boot factory chimney. There were a few other tall chimneys as well. He'd never seen those before. The man with the dog was watching them from some way off. So was the dog. Neither of them seemed particularly Jurassic, although the dog looked somewhat suspicious.

"Wha..?" said Wobbler. "Here, what's been happening? What have you done?"

"I told you we'd travelled in time," said Kirsty. "Weren't you listening?"

"I thought it was just some trick! I thought you were just messing about!" He gave Johnny a very worried look. "This is just messing about, isn't it?"

"Yes."

Wobbler relaxed.

"It's messing about with time travel," said Johnny.

Wobbler looked scared again.

"Sorry. But that's Blackbury all right. It's just smaller. I think we're where the mall is going to be."

"How do we get back?" said Yoless.

"It just sort of happens, I think."

"You're just doing it with hallucinations, aren't you," said Wobbler, never a boy to let go of hope. "It's probably the smell from the trolley. We'll come round in a minute and have a headache and it'll all be all right."

"It just sort of happens?" said Yoless. He was using his careful voice again, the voice that said there was something nasty on his mind. "How do you get back?

"There's a flash, and there you are," said Kirsty.

"And you're back where you left?"

"Of course not. Only if you didn't move. Otherwise you go back to wherever where you are now is going to be then."

There was silence while they all worked this out.

"You mean," said Bigmac, "that if you walk a couple of metres, you'll be a couple of metres away from where you started when you get back?"

"Yes."

"Even if there's been something built there?" said Yoless.

"Yes ... no ... I don't know."

"So," said Yoless, still speaking very slowly, "if there's a lot of concrete, what happens?

They all looked at Kirsty. She looked at Johnny.

"I don't know," he said. "Probably you kind of ... get lumped together."

"Yuk," said Bigmac.

There was a wail from Wobbler. Sometimes, when it involved something horrible, his mind worked very fast.

"I don't want to end up with just my arms sticking out of a concrete wall!"

"Oh, I don't think it's happen like that," said Yoless.

Wobbler relaxed, but not much. "How would it happen, then?" he said.

"What I think would happen is, see, all the atoms in your body, right, and all the atoms in the wall would be trying to be in the same place at the same time and they'd all smash together suddenly and-"

"And what?" said Kirsty.

"-and ... er ... bang, goodnight, Europe," said Yoless. "You can't argue with nuclear physics, sorry."

"My arms wouldn't end up sticking out of a wall?" said Wobbler, who hadn't quite caught up.

"No," said Yoless.

"Not a wall near here, anyway," said Bigmac, grinning.

"Don't wind him up," said Yoless severely. "This is serious. It could happen to any of us. We dropped when we landed, right? Does that mean that if we suddenly go back now we'll be sticking out of the floor of the mall, causing an instant atomic explosion?"

"They make enough fuss when you drop a Coke tin," said Johnny.

"Where's Wobbler gone?" said Kirsty.

Wobbler was a disappearing shape, heading for the allotments. He shouted something.

"What'd he say?" she said.

"He said "I'm off home!" said Johnny.

"Yeah, but," said Bigmac, " ... where he's running now ... if we're where the mall is ... will be ... then over there's the shopping estate. That field he's running across." He squinted. "That's where Currys is going to be."

"How will we know we're about to go back?" said Yoless.

"There's a sort of flicker for a moment," said Johnny.

"Then ... zap. Er ... what'll happen if he comes out where there's a fridge or something? Is that as bad as a concrete wall?"

"I don't know much about fridge atoms," said Yoless. "They might not be as bad as concrete atoms. But I shouldn't think anyone around here would need new wallpaper ever again."

"Wow! An atomic Wobbler!" said Bigmac.

"Let's get the trolley and go after him," said Johnny.