Being the new kid at school is hard, but Sophie has a secret weapon: her vivid imagination and her oversized, trusted Big Book of Monsters, an encyclopedia of myths and legends from all over the world. The pictures and descriptions of the creatures in her book help her know which kids to watch out for—clearly the bullies are trolls and goblins—as well as how to avoid them. Though not everyone is hiding a monster inside; the nice next-door neighbor is probably a good witch, and Sophie's new best friend is obviously a good fairy.

Sophie is convinced she is a monster because of the monster mark on her face. At least that's what she calls it. The doctors call it a blood tumor, and it covers almost half of her face. Sophie can feel it pulsing with every beat of her heart. And if she's a monster on the outside, then she must be a monster on the inside, too. She knows that it's only a matter of time before the other kids, the doctors, and even her mom figure it out.

The Big Book of Monsters gives Sophie the idea that there might be a cure for her monster mark, but in order to make the magic work, she'll need to create a special necklace made from ordinary items: a feather, a shell, and a crystal that Sophie believes are talismans. Once she's collected all the needed ingredients, she'll only have one chance to make a very special wish. If Sophie can't break the curse and become human again, her mom is probably going to leave just like Dad did. Because who would want to live with a real monster?