Jack, Mika and Alex run after Billie, down the stairs, through the kitchen, out the back door and into Jack’s garden. One by one, they squeeze through the hole in the fence and into Billie’s garden.

Billie waits impatiently for them at the back door to her house. ‘Dad’s baking!’ she says. ‘And I’ll bet they’re lemon biscuits.’

The others look at each other, confused.

‘Come on!’ says Billie, leading them into the kitchen. The oven is on, but her dad has left the room. She can hear him talking to Noah upstairs.

Billie runs to the oven and flattens the paper onto the warm glass.

Nothing happens for a moment. Billie holds her breath, crossing her fingers. Then slowly, as they watch, pale brown letters and shapes begin to appear across the page.

Mika gasps. ‘How did you do that?’

‘It’s written in lemon juice,’ Billie explains. ‘Dad and I saw it on a science show once. When the lemon juice heats up the writing turns brown.’

As they stare at the paper five letters and a drawing appears:

‘What does that mean?’ Alex says.

‘Well, that’s an apple tree, obviously,’ Billie says. ‘SMC is us. And HQ stands for headquarters.’

‘Secret Mystery Club Headquarters apple tree?’ Jack says, frowning.

Billie dashes out the back door again. ‘Follow me!’ she yells over her shoulder.

She runs all the way to the end of her garden, with the others close behind.

There, by the chook pen, is their enormous apple tree. Dangling from its branches is a long, thin cord.

Billie pulls it, and a rope ladder comes tumbling down. She squeals in delight.

Billie scrambles up the ladder and gasps when she sees what is hidden there.

‘It’s a treehouse!’ she yells to the others down below. ‘Oh, wow, it’s brilliant! Come up — it’s the coolest thing you’ve ever seen.’

One by one, the others climb up the ladder to the platform of the little wooden treehouse.

Billie feels like her heart might burst with happiness.

It’s the most perfect treehouse she could ever imagine, just big enough for four people, and hidden away from the world. ‘This can be our clubhouse!’ Billie says. ‘The SMC headquarters. Just like it said on the note.’

‘This is incredible!’ Jack says.

‘It’s beautiful!’ Mika sighs.

‘Amazing!’ says Alex. ‘Who could have built it?’

Billie peers out through the leaves across her back garden.

From where she is sitting she has a perfect view of their kitchen. Through the windows she can see her dad inside, taking the lemon biscuits out of the oven.

At that moment he looks up and gives a little wave. Billie smiles. ‘I think I know,’ she says.

Jack looks down and sees Billie’s dad too. ‘Your dad?’

Billie nods. ‘He’s been acting very weirdly lately. And he borrowed tools from Alex’s dad the other day.’

‘But how could he have done it without you noticing?’ Jack asks.

‘Well, I’ve been kind of busy,’ Billie says. ‘Following a trail of secret notes. And cracking codes!’

‘What? You think he wrote all those notes, too?’ Mika says. ‘But he can’t write Japanese!’

‘No, but your mum can,’ Billie says. ‘And Jack’s dad is very good at backwards writing.’

‘Oh!’ says Alex. ‘You think all our parents were in on it?’

Billie shrugs. ‘Dad was on the phone to your parents a lot this week.’

It is all coming together now in Billie’s mind.

‘But, hold on,’ Alex says, frowning. ‘How did your dad even know about our club? I thought we had all sworn not to tell anyone!’

Billie looks at Jack. His cheeks turn very pink.

‘That was my fault,’ he says quietly.

‘I got scared when Billie was talking about the Spooky House.

I told my mum about it. She must have told Billie’s dad.’

‘It’s not your fault,’ Billie says. ‘It was my fault for scaring you. And besides, if my dad hadn’t found out about the club he might never have built us this amazing clubhouse!’

‘Or organised all those codes to crack!’ Mika agrees.

‘Yeah, I guess it doesn’t matter,’ Alex says. ‘As long as it’s only our parents. But no one else, OK?’

‘OK!’ everyone agrees. Then Jack does the SMC rooster call just to seal the deal and everyone laughs.

‘It doesn’t feel like we solved a real mystery though,’ Mika sighs. ‘Now we know our parents set it up.’

‘Yeah, I wish we had a real mystery to solve,’ Alex agrees.

They all look at Billie.

Billie is thinking about something. She picks a leaf from the apple tree and rolls it between her fingers.

Finally she turns to them. ‘I think I have the perfect mystery,’ she says, smiling. ‘Meet me in the playground before school tomorrow.’

‘What is it?’ says Alex, eagerly.

‘You’ll have to wait and see,’ says Billie. ‘But don’t worry. You won’t be disappointed.’

To be continued …