The novel is based on an ancient legend about a dragon that sleeps under the medieval castle of the Zrinski family in the small north-Croatian town of Čakovec. The dragon is so big that his head is under the castle and his tail under the church in town centre. Four children from diverse backgrounds discover a secret passage that leads to a huge underground cave with a skeleton of a dragon and an egg in which something seems to be alive. After their discovery, the children are chased by mysterious organizations bent on finding the secret passage. The unlikely friends use a combination of cunning and modern technologies (drones, computers, smartphones) in their fight and eventually form a firm bond that enables them to defeat their enemies and protect the egg. The novel uses topics and tropes from various ancient Slavic legends about dragons, werewolves, witches and pagan gods, and creates a world in which folk-tales are closely intertwined with our modern society.