Nine months after the events of Tree Magic, Rainbow is struggling to come to terms with how she's changed. Her bond with trees has grown, but now they're under threat from the Tree Slayer. To save them, she will need to leave everything she knows and loves behind. It'll be her greatest adventure yet – but she cannot succeed alone. She will have to enlist the help of Eole, an enigmatic scientist and sceptic. But does Rainbow have what it takes to face the Tree Slayer? And can she trust Eole, who has powers of his own?