Edwin really doesn't want to be a big brother. Forced to move house, start a new school and make way for this unwelcome interloper in his parents' affections, he feels like everything is chaos. But things might not be as bad as he fears, as Edwin makes an unusual pen-pal, Lanthorne, who introduces him to a strange world filled with dark secrets and thrilling adventures.

This excitement seems safe until those secrets and adventures start to intrude on Edwin's life and, more worryingly, on the life of his baby sister. Can Edwin and Lanthorne work together to save Mandoline from the evil Aunt Necra? And can they figure out why Lanthorne's family is so divided, and what the terrible secret is that so many people seem to know all about but refuse to share with Edwin?