Henrietta and her pals Gary and Rose all have headaches with an unknown cause and experience other strange incidents--including the appearance of a threatening creature called the Wikkeling--and are hopeful that an ancient bestiary can help solve these mysteries. In the city of Addition, where the children are safe and watched by cameras day and night, Henrietta, Gary, and Rose suffer from mysterious headaches, a wounded cat turns up in the attic, and a variety of other strange things occur.

Contains black and white silhouette illustrations, and a section with a Bestiary containing full-color illustrations and varied typography.