Tobie Lolness is 11 years old and a millimetre and a half tall, which is not very tall for his age.

Toby escapes with his family. They have flee urgently because Sim Lolness removed the heart of Balina’s engine before he was banished and a furious Jo Mitch has sent his men after them. Our heroes make it to the Low Branches by the skin of their teeth, fake their own deaths, and go into hiding.

One year later, Maya Lolness gets a letter from Mano Asseldor telling her that her mother, Radegonde Alnorell, is very ill. Despite the dangers of the journey, the Lolnesses decide to travel up to the Treetop. Toby is longing to see his friend Leo again and insists on coming along. He says goodbye to Elisha and promises to come back soon.

Speaking at the Tree Council, Jo Mitch accuses Sim Lolness of selling the secret of Balina to the Grassies and contrives to have him sent to Tumble Jail. He organises a massive manhunt for Toby with the promise of a big reward. Leo refuses to believe the accusations but is troubled nonetheless. Toby takes refuge in the Lichen Woods, where he comes across Nils Amen.

Toby gets away from Leo, only to fall into Pinhead’s clutches. To his horror, he discovers the Crater, a raw, gaping wound in the tree made by dozens of weevils digging insatiably for its lifeblood sap. Toby outwits Pinhead and manages to escape, but a conversation he overhears about some poor person called “Thing” who is mercilessly bullied by Jo Mitch’s men makes him turn back to rescue him.

The two boys embark on a perilous dash for survival, in which Mano has to overcome his cowardice, own up to his lies and show great courage. Together, they narrowly escape deadly mosquitoes, battle against a spider and eventually reach the Low Branches, where Toby delivers Mano back to his family.