As soon as the bell rings, Billie runs out into the playground with her friends.

‘I’ve got it! I’ve got it!’ she yells as soon as they have reached the peppercorn tree. ‘I’ve cracked the code!’

‘So have I!’ Alex says. ‘And I’ve already worked out the message. I did it in class. See?’ He unfolds the paper. There, underneath the rows of numbers, is his neat handwriting:

Billie’s heart sinks like a stone. ‘How did you work it out?’

Alex shrugs. ‘It was easy. It’s an alphabet code. 1 is A, 2 is B and so on. It didn’t take me long. And I’ve already worked out what the red train set is. It’s an old train set of my dad’s in our toy cupboard. That’s where the next clue will be.’

‘Good work!’ says Jack, patting Alex on the back.

‘Yeah, it was pretty clever to work that out, Alex,’ Mika says. ‘Wasn’t it, Billie?’

‘Uh-huh,’ Billie says quietly. She knows she should be happy that they have worked out the code, but she can’t help it. She was going to crack that code.

Billie feels like she might cry. While the others are looking at the note again, she walks quickly away.

Billie goes to the monkey bars and climbs up to the very top. There, she can let the big salty tears stream down her face without anyone seeing them.

Before very long, a head pokes over the top of the monkey bars. It is Jack, pulling a funny face.

Billie wipes her eyes on her sleeve. Even though she is feeling mad and sad and all kinds of horrible feelings bunched up together, she still can’t help smiling when she sees Jack’s silly expression.

He climbs up and sits beside her. ‘What’s up?’ he says.

Billie scrunches up her face and sniffs. ‘It’s stupid,’ she says, embarrassed. ‘You probably won’t even want to be my friend anymore if I tell you.’

‘Billie!’ says Jack, rolling his eyes. ‘Haven’t we been friends forever?’

Billie nods.

‘Haven’t we told each other all of our secrets?’

Billie nods again.

‘Well?’ Jack says, raising his eyebrows and waiting.

Billie takes a deep breath. She looks down at the playground and sees Mika and Alex sitting together under the peppercorn tree.

‘I feel left out,’ she says in a little voice. ‘All of you have been able to work out the codes except me. Now I don’t feel like I should be a member of the Secret Mystery Club any more because I’m such a bad detective.

Alex and Mika probably don’t even want me in the club now.’

Billie knows this last bit isn’t really true. Alex and Mika are her friends too. But she really wants Jack to understand how upset she is feeling so she decides there is no harm in exaggerating just a teensy bit.

‘What are you talking about?’ Jack says. ‘I haven’t cracked a code yet either. Only Mika and Alex have. And they were just lucky. Of course we want you in the club! There wouldn’t even be a Secret Mystery Club without you!’

‘I guess so,’ Billie sniffs. It is true that Jack hasn’t cracked a code yet either. She had forgotten that.

Jack thinks for a minute. ‘How about we ask Mika and Alex if when we get the next code, you can have first go at cracking it? That would be fair.’

Billie smiles a little smile. ‘Thanks, Jack,’ she says. ‘You’re a good friend. I know I’m just being silly.’

Jack grins. ‘If you don’t stop being grumpy, I’ll be your only friend!’ he jokes.

Billie giggles. She swings her legs happily in the empty space beneath them, her heart feeling warm again. ‘Who do you think is sending us all these weird notes, anyway?’

‘No idea,’ Jack says, swinging his legs alongside her. ‘It’s fun though, isn’t it?’

‘Sure is,’ Billie says. She can’t wait to crack tomorrow’s code.