Magic Detective Agency #10 -- the Trek

Magic Detective Agency occasionally encountered the young help seeker, and the young help seeker said that he had seen monster that ate people. He could describe macroscopic features of monster in detail.Although small aides thought that the young help seeker was playing a trick, the doctor believed that he was not lying. He decided to find reasons to the place where the accident happened with his aides.What is the relation between Liboson River where the missing person case happened and surrounding mysterious Cape Castle?What is the relation between monster and the both?How did members in the magic detective agency reveal the mystery, and how they capture monster?