A Postcard to Ollis

Ollis' world has been turned upside down. She's got a new baby brother and, even worse, her mum's neat-freak boyfriend has moved in. Fortunately Ollis has always got her best friend Gro, as she lives right next door and they know each other inside out. Or do they? Ollis hasn't always been completely honest with Gro and when they find a postcard in a mysterious yellow mailbox in the forest, Ollis' lie grows even bigger. What does Ollis know about the postcard that Gro doesn't?