The Secret Mystery Club decide they can’t wait until the next day to find the letter, so they all ask their parents if they can go to Jack’s house after school.

As soon as they arrive, the four of them run up to Jack’s room. Sure enough, in the dust and gloom under his bed, a little white envelope shines brightly.

Jack pulls it out and hands it to Billie. ‘The last letter!’ he says.

The others all gather around Billie, waiting.

‘Open it! Open it!’ Mika begs.

Billie sits cross-legged on the floor with the envelope in her hands. Written across the front in pale blue writing, are the words:

Her heart is flipping about like a bird. What if I can’t crack this code? she worries. Then she reminds herself: Of course I’ll be able to! This one’s for ME!

With trembling hands, Billie pulls open the seal and reaches inside the envelope for the slip of paper. Then she unfolds it and smooths it on the floor in front of her.

Everyone gasps.

There is nothing written there. Nothing at all.

‘I can’t believe it,’ Jack says, quietly. He puts his hand on Billie’s shoulder.

Billie stares down at the paper. She can’t believe it either. How could the very last note — her note — have nothing written on it? Is this an awful trick? Is there nothing at the end of this trail of notes after all?

But then an idea comes to her.

Someone knew Mika could read Japanese, she thinks. And that Alex was good with numbers and Jack could read backwards. She remembers her dad telling her that every detective has different skills.

Suddenly, Billie has the answer. She looks up at her friends. ‘Don’t worry!’ she says, grinning. ‘I think I know how to read the note. Follow me!’ She stands up with the letter in her hand and dashes out of Jack’s room.