The time is what the military call 'oh-three-hundred-hours' - three o'clock in the morning. We're in the car park of the sports centre. It's totally empty except for me, my best friend, Janey and an old truck that belongs to the company that put grass on the town football pitch last week.

'What are we doing here?' I ask.

'Here's the plan,' says Janey. 'First, we borrow that truck ...'

'I'm going home,' I say. 'Good night.'

'Then, we drive to the Town Hall. And then we use the grass in the truck to cover the high street from top to bottom, like one great big garden. Ready?'

I think about it for a minute. Then I smile.

'OK,' I say. 'Let's go.'

...

Our new green high street is beautiful. But we don't have long to admire it. The judge says we're guilty of a crime. Several crimes in fact, including theft, vandalism and disturbing the peace. I get sentenced to 150 hours of community service and Janey gets 300 hours because it was her idea.

I stay out of trouble, do 150 hours cleaning graffiti and get back to my studies. Janey is planning how we're going to do our guerrilla gardening better in future. Our mission is to make the whole town green again. Janey's ideas will probably put us both in prison eventually. But someone needs to take urgent action to help the planet.