Mari loves her grandfather Peeter, who comes up with the most ingenious treasure hunts for her and her three friends. They’ve just found “Marie Antoinette’s gloves” when they decide to found a secret society and are confronted with a real adventure. During the summer fair, a mysterious masked man spiked the free drinks with a poison. The fatal effect of the toxin is that most of the adults suddenly behave like naughty children. Only grandfather Peeter knows what’s going on. He gives Mari an old notebook that allegedly shows the way to where the antidote is hidden. The four friends set out on an adventurous trip full of riddles, where they also have to keep the masked man and Leo’s gang at bay. An imaginative film, with historical allusions and bicycle stunts that are slightly reminiscent of Indiana Jones.