Told as a memory recounted by a grown man, the story follows a young boy who wakes disoriented in the crumbling ruins of a vast, ancient valley known as the Nest, with no memory of how he arrived. There he finds Trico, an enormous creature that is part bird and part mammal, chained and wounded. Though the beast is wary and frightened at first, the boy earns its trust by pulling the spears from its body and bringing it food, and a fragile bond forms between them. Together the unlikely pair set out to escape the labyrinth of towers, bridges, and caverns, relying on Trico's great strength and the boy's cleverness to survive the dangers that haunt the ruins.