That evening while Billie and her family are having dinner, there’s a knock on the back door.

‘I’ll go!’ says Billie. ‘It’ll be Jack.’

Billie’s mum nods. Jack is the only visitor who comes to the back door.

Billie opens the door and grins when she sees Jack standing on the back step.

‘Hi, Jack!’ Billie’s mum calls.

‘Hi!’ Jack calls back. Then he whispers to Billie, ‘Can you come outside for a second? I’ve got something to show you.’

‘Sure!’ says Billie, feeling her tummy flip excitedly. ‘What is it?’

Jack leads Billie outside. It is already dark.

In the shadows, Billie sees something leaning against the back wall.

‘The metal detector!’ Billie squeals. ‘How did you get it so quickly?’

Jack grins. ‘I told my dad that Mika lost her necklace in the playground and needs to find it before her mum notices it’s gone. So Dad rang Uncle George, who brought it over just now.’

Billie sees Jack’s cheeks turn a little pink. ‘I hope it’s OK that I told them the made-up story?’ he asks.

‘Detectives have to do that sometimes,’ Billie assures him. ‘Especially when they’re working undercover. It’s not exactly lying. It’s just not telling the whole truth.’

Jack looks relieved. ‘Should we take it to school tomorrow? There won’t be anyone there on a Saturday.’

‘Good idea!’ says Billie. ‘I’ll call Alex and Mika after dinner and tell them to meet us there. Eet-may ou-yay at-ay ool-skay at-ay en-tay.’

Jack scrunches up his brow. ‘What?’

‘Meet you at school at ten!’ Billie giggles. ‘It’s Pig Latin. Dad taught it to me. He used to use it when he was a kid. I’ll teach you guys tomorrow. It’s perfect for detectives.’

‘Billie!’ calls her dad from inside. ‘Your dinner’s getting cold.’

‘Coming!’ Billie calls back. ‘See you tomorrow, Jack!’

She does an excited little skip to the back door. This is the best mystery ever! she thinks happily.