In the summer, ice cream was sold in a booth on Plazowa Street, and now - just before Christmas - someone decided to sell gingerbread there. When the young detectives come there again to shop, they find a 'I'LL BE BACK SOON' sign. Something's wrong. Will the seller come back? Why did he use a booth close to a closed house? When Tiril and Oliver notice a hole cut in the back wall, they spring into action...