Billie B Brown sits with her friends in their brand-new Secret Mystery Club Headquarters. It is a treehouse that seems to have appeared out of nowhere, just like magic, in the old apple tree at the end of Billie’s backyard.

One day, there was nothing in the tree but an old bird’s nest. The next, there was the most brilliant treehouse Billie and her friends could ever imagine.

Billie knows it was her dad who built the treehouse. She also knows it was her dad who organised the trail of notes with secret codes, to keep them busy while he was building it.

All their parents were in on it.

Each of them came up with a code they knew only their child would be able to crack.

Even though the treehouse is the most wonderful surprise ever, Billie and her friends have decided it wasn’t a real mystery if their parents made it up. It was more like a fun game.

But Billie has had an idea. A super-dooper idea. There is something at school she thinks the SMC should investigate.

‘Please tell us, Billie!’ Mika begs. ‘We can’t wait until tomorrow. Go on! Just give us a clue.’

‘Well, all right,’ Billie says, grinning, and she draws the others in close to listen. ‘I guess I can tell you now. You remember how Benny got called into the principal’s office last week for putting leaves in Rebecca’s sandwich?’

Jack, Mika and Alex all nod.

‘Well, I don’t think it was him,’ Billie says. She crosses her arms and smiles triumphantly, waiting to see what the others will say.

‘Of course it was him!’ Alex snorts. ‘Benny always does stupid things like that.’

‘And Lola said she saw him in the corridor by the school bags that day,’ Mika adds. ‘When she was going to the bathroom. So there’s even a witness.’

‘Yes, but did she actually see him put the leaves in Rebecca’s sandwich?’ Billie asks. ‘She’s only a witness if she actually saw him do it.’

‘No, but his pockets were full of mulberry leaves,’ Alex says, reminding Billie. ‘He was pretty much caught red-handed, Billie. Case closed.’

‘Yeah, that’s definitely evidence,’ Jack says. ‘He had leaves in his pockets, Billie!’

‘Maybe,’ says Billie, staring mysteriously into the distance. ‘But he still says he didn’t do it. He says the leaves were for his silkworms.’

‘Wouldn’t you say you didn’t do it?’ Alex says.

Billie shrugs. ‘I might, but not Benny. He loves boasting about all the silly things he does. He thinks it makes him look cool. But this time … he seemed different.

Didn’t you notice? He was almost in tears when Mr Benetto sent him to Mrs Singh’s office. He kept saying, “I didn’t do it! I didn’t do it!” That’s not like him at all.’

Mika nods slowly. ‘You have a point,’ she says. ‘I’ve never seen Benny cry before. Not even that time he broke his arm at soccer.’

‘It’s true,’ says Jack. ‘But who else would put leaves in Rebecca’s sandwich? It’s a pretty stupid thing to do.’

A slow smile creeps over Billie’s face. ‘Well? Isn’t this a mystery we can solve?’

‘That’s your super-dooper mystery?’ Alex says, looking unimpressed. ‘I thought it would be a bit more exciting than that!’

‘I think it’s a good idea!’ Mika says, standing up for Billie. ‘If someone got in trouble when it wasn’t their fault, isn’t it important to find out who really did it?’

‘Yeah,’ says Jack. ‘It’s not fair if Benny got in trouble for something he didn’t do.’

‘And now Rebecca’s not talking to him,’ Mika says. ‘Neither is Lola.’

‘What do you think?’ Billie asks looking hopefully at Alex. Even though he is annoying sometimes, she still wants him to like her ideas.

‘I guess you’re right,’ says Alex.

‘Then we start our investigation tomorrow!’ Billie says happily.

She sticks her hand into the middle of their circle. Jack slaps his hand down on hers, then Mika, then Alex. ‘Cock-a-doodle-dooooo!’ they all shout together.