Shallot Public School has a secret - a dark secret that 12-year-old Annie is about to step right into the middle of. The eighth grade should be easy for her. She's smart, a big-hearted dreamer and hopeful writer, but dreams and passion can't possibly prepare her for what she encounters when she enters Miss Henshaw's class on the first day of school. 

Miss Henshaw, her favorite teacher, isn't there at all and instead has been replaced. The new teacher, Miss Grundy, takes an instant dislike to Annie and her sunny disposition, so much so that she decides to teach Annie a lesson she'll never forget and in the process destroys Annie's happiness. 
Soon after Annie's friends don't trust her, her family is upset with her, too, and she doesn't have the inspiration to write anymore. Just when Annie has lost all and doesn't know what to do, she encounters a very special mystical creature called a muse. She is from Olympus who comes down to earth to warn Annie about Ms. Grundy's real identity. Ms. Grundy is a gorgon, a malicious she-demon with plans to steal humanity's creativity, taking away their hope and turning them into her willing slaves. 
Tammy the Muse has a secret of her own and has gone against the wishes of Zeus, the King of Olympus, to never interfere in the affairs of mortals, however,  she knows that only Annie, with her incredible imagination, can stop Ms. Grundy, but she can't do it alone. To save her loved ones, Annie must find her courage, unite a disparate school against the darkness that is Ms. Grundy and, in the end, save Tammy from the wrath of the gods.