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!