Oliver & Daniel: the Invisiblity Cloaking Device (The Netherworld Series) (Volume 3)

Oliver & Daniel return to Netherworld, where they hope to meet up with Tania again and explore Estopia, a strange medieval town which they briefly saw on their last visit. But there is a time difference between the portal they use this time and the one they went through previously. As a result, they arrive just in time to see themselves leave from their last visit and narrowly avoid the embarrasing, and potentially dangerous situation, of meeting themselves. Estopia is being terrorised by an evil man who is robbing the people using an invisibility cloaking device to conceal himself. Returning home through the wrong portal by mistake, they meet their younger sister, now 9 years old, and discover that she has never had any brothers! They must not only deal the evil Fagiola but also solve the mystery of why they have disappeared from the future, leaving their young sister with no brothers!