Mrs Peterson gives her class a big new project - they have to find something they can sell at the Tunbridge Town Fete. At first, Justin's group don't know what to sell, but then Justin has a bright idea - they'll sell 'worm juice'. Worm juice is a wonderful fertiliser for plants! But Justin and his friends soon discover that setting up a worm farm isn't easy, and as the date of the fete gets closer, they wonder if they're going to have anything to sell!