For the very first time, Conni goes on a big trip without her family. Of course her best friends Anna and Simon are joining, but unfortunately she is not allowed to take Cat Mau with her. As they arrived in the hostel, an old mill close to a castle, Conni discovers that Mau had come as a blind passenger. Secretly she cheers, although she knows that animals are forbidden in the castle and the hostess, Ms. Weingartner, is allergic to cats. Conni tries to hide Mau, but the cheeky cat cares little. He explores the surroundings curiously and does a lot of nonsense. Ms. Weingartner identifies her son's pet, the sweet raccoon Oskar, as the culprit and wants to take him to the zoo. And then things disappear from the hostel in a strange way. Conni and her friends have to act quickly: Will the children be able to save raccoon Oskar, tame Mau and also find a real thief?