A family vacationing in the Burlington Library is attacked by a shadowy fiend wearing a head similar to that of Anubis and is scared off when it assumes a werewolf-like form. Later, a landslide forces the gang to abandon their ski vacation and detours them to the same library, where they are greeted by its caretaker Dan Fluunk who explains to the gang the history behind the mansion, including that its builder and namesake had a fascination with groups of four people and an animal. Soon, strange things begin happening to the gang as their stay in the mansion progresses. Eventually, their experiences are revealed to be hallucinations brought on by the burning of a special kind of wood called Terror Wood. Meanwhile, Cassidy Williams continues her search for the truth behind what is happening to the gang now and what has happened to other mystery-solving groups in the past — including the Darrow family.