‘Billie!’ Billie’s mum calls to them from the back door. She has Noah on her hip. ‘Mika’s mum is here to take Mika and Alex home.’

The four of them climb down out of the treehouse one by one, landing on the grass with soft thuds.

Billie looks up at the little house again and smiles. From the ground it is completely hidden by the leaves. She feels her heart almost burst with pride.

‘Let’s meet in the playground before school tomorrow to discuss our plan,’ Billie says to the others. ‘I’ll bring my special secret notebook. We can write down our ideas in there.’

‘Oh! I have a little digital voice recorder!’ Alex says, remembering.

‘If we interview people we can record what they say.’

‘Good thinking, Alex!’ Billie says.

‘Come on, let’s go inside. But remember, everyone, this is top secret—OK?’

‘OK,’ agree the others and they run inside.

After Billie’s friends have left, Billie goes upstairs to find her dad. He is in the bathroom giving Noah a bath.

Billie’s little brother is splashing about so much that her dad is covered with water and bubbles.

Billie giggles when she sees him.

‘So,’ she says, leaning against the doorframe. ‘This treehouse just appeared in our apple tree.

Out of nowhere!’

‘Really?’ says her dad, raising his eyebrows. ‘How astonishing!’

‘What house? What house?’ Noah asks.

‘A clubhouse. For a secret mystery club,’ Billie says, grinning.

‘I have no idea what Billie is talking about,’ her dad says to Noah. His mouth twists up a little in the corners. ‘I’ve never even heard of a secret mystery club. Have you, Noah? If it’s a secret mystery club, how can I possibly know anything about it?’ He winks at Billie.

‘What secret? What secret?’ Noah says. ‘Can I have a house-club-tree-secret?’

Billie and her father laugh. Billie wipes some bubbles from her dad’s cheek and gives him a big kiss. ‘Thanks, Dad,’ she whispers. ‘You’re the best.’

Her dad winks and dabs some bath foam onto her nose. ‘You’d better go. Otherwise you’ll get covered in bubbles, too!’

Noah squeals and tosses a fistful of foam into the air. Billie ducks away quickly and jogs back downstairs.

Yummy dinner smells are coming from the kitchen, mixing with the lovely lemony smell of the biscuits her dad baked that afternoon.

Billie pulls a stool up to the kitchen bench and perches beside her mum, who is grating carrots. ‘What’s for dinner?’ she asks.

‘Spaghetti bolognaise,’ her mum answers, smiling.

‘My favourite!’ Billie says, happily.

At that moment she feels like the luckiest girl in the world. She has a secret treehouse, three best friends and a new secret mystery to solve.