‘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 Yo-less.

‘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 Yo-less.

‘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 Yo-less 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 Yo-less. ‘Ah. 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 top.

‘If I asked what happened,’ said Yo-less, 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 Yo-less, still talking in his deliberate voice. ‘Are we talking dinosaurs, or mutant robots? I want to know this before I open my eyes.’