When Arch turns up at St Grizzle's having run away from home, Dani is delighted to introduce her best friend to her new classmates, only to discover that mixing her old life with her new one isn't quite as easy as she'd imagined. And Arch isn't the only new pupil - super-smart Boudicca arrives, and in spite of everyone's best efforts to make her feel welcome, she barely says a word to anyone and has a habit of vanishing. It's only when she meets Arch that Boudicca perks up and takes to following him around everywhere. Then she disappears. With the help of Granny Viv, it's up to Dani and Arch to find Boudicca ... and unravel the truth about this shy tag-along zombie.