One thing you may not know about me is that I used to be a detective. From about seven to nine years old, my friends and I kept a faithful watch over our neighbourhood, to make sure things were running just the way they should. At this time, I was living in Perth, Western Australia, in the supposedly quiet suburb of Mount Lawley, but you have no idea how many astonishing things went on under the inattentive gaze of our parents; things that went missing, unexplained noises in the night, mysterious stains on our clothing that would appear out of nowhere. I can tell you, if my friends and I hadn't been keeping careful watch over our parents and younger siblings, who knows what tragedies may have struck.

Obviously, when you're a detective, you can't have everybody knowing this. It's very important to remain undercover and to find ways to communicate with your fellow neighbourhood detectives in a way that is indecipherable to others. So, my friends and I would develop new languages and complex codes, that sometimes became too baffling to even decrypt ourselves.

Something as simple as Meet Me In The Park After School would become:

LOOHCS RETFA KRAP EHT NI EM TEEM

or

Teemay emay inay ethay arkpay afteray oolschay

or, worse still (because numbers were never my forte)

13-5-5-20 13-5 9-14 20-8-5 16-1-18-11 1-6-20-5-18 19-3-8-15-15-12

Often, to my chagrin, by the time I had cracked the code the meeting would be well and truly over.

Fortunately, Billie and her friends work together a little more constructively than my friends and I ever did and manage to crack a series of curious codes that appear one after the other in little envelopes all around their neighbourhood. The code in each envelope relies upon a child's particular skills; Mika's ability to read other languages, Alex's talent for numbers and Jack's technique for memorising tricky words. Billie begins to despair that she may never crack a code herself, but the last note they find requires a particularly imaginative thinker and Billie is able to work it out, thanks to a few clues she has gleaned along the way. They follow its instructions which lead them to discover something completely and unexpectedly glorious, hidden in Billie's very own backyard. Something just perfect for a Secret Mystery Club to ponder their next impending mystery.