Quiet, brilliant Portia has just moved to a new neighborhood with her mom. Adjusting to life without a father is hard enough, but school is boring and her classmates are standoffish -- and even Portia's mom is strangely distant. But things start looking up when Portia mounts a late-night excursion into the woods behind her house and discovers a shy, sweet-natured purple monster. Life with Jellaby is a lot more exciting, but Portia's purple friend has secrets of his own; secrets that may even lead to the mystery of Portia's father's disappearance!