The next morning, Billie arrives at school extra early and waits for Jack, Alex and Mika under the peppercorn tree in the playground.

She takes her notebook, which has a real lock, out of her school bag and opens it up to a clean page.

Across the top of the page she writes the date in curly writing with her blueberry-scented pen. Then she draws a wiggly line and some stars underneath for decoration.

Her notebook is so small that now almost the whole page has been filled, so she writes in extra-small writing:

SMC Mystery number 3: Who put leaves in Rebecca’s sandwich?

When she looks up again, her friends are jogging towards her across the playground. They sit down beside her and look over her shoulder to read what she has written in her special secret notebook.

‘So, what’s the plan?’ Jack asks.

‘Well, I think we should start by asking people questions to find out some clues,’ Billie says. ‘But don’t make anyone suspicious, OK?

We don’t want them to know we are investigating a mystery.’

Jack, Alex and Mika nod.

‘Why don’t I talk to Benny?’ says Billie. ‘And see if I can get any more information out of him.’

‘Good idea,’ says Mika. ‘I’ll talk to Rebecca at lunchtime. We’re on compost duty together today.’

‘Great!’ says Billie.

‘I’ll talk to Lola,’ says Jack. ‘She’s the main witness!’

‘And I’ll talk to Sam,’ says Alex. ‘He’s Benny’s best friend so I’m sure he’ll know something.’

Then Alex pulls a tiny voice recorder out of his pocket and grins. He presses a button and all of their voices come out of it, high and squeaky. ‘You didn’t even know you were being recorded, did you?’ he says gleefully. ‘Isn’t it cool?’

‘Wow, sure is!’ says Jack. ‘Can I see?’

Alex shows Jack how to work the recorder, and the two of them record each other making funny voices. Billie and Mika giggle.

Just then the bell goes. ‘Oh no,’ says Billie. ‘We still haven’t worked out our whole plan.’

‘I think we have enough to start with,’ Alex says. ‘Let’s interview people today, then why don’t we meet in the treehouse after school and we can share anything we’ve found out?’

‘Good idea,’ says Billie. ‘Will your parents let you come over?’

‘I think it should be fine,’ says Mika. ‘My mum can take Alex home again afterwards.’

‘Great!’ says Billie. ‘Then, let’s get to work. And meet you at the treehouse after school!’

The four of them run off to class.