Torus: The Story of the Tenth Stone is the fourth book in the Ocellus series, and takes place on Squan, a mountainous Peru-like planet with three moons. The events recorded take place billions of years before those recounted in the first book in the series: Ocellus: The Story of the Twelfth Stone.On their way home from school on a high mountain path, Pynxle (aged 11), along with her twin brother Cheo, and two older school friends, finds an oval glassy-black stone with a torus carved on one face. Immediately, a friendly shapeshifter, Septies, confronts them, saying he has searched for the Torus stone for trillions of years.Pynxle, however, is reluctant to relinquish the stone and soon after, an evil shapeshifter, XorX, attacks them and demands to be given the Torus stone. To protect the children and the stone, Septies chivvies them into a cave for safety and confronts XorX.In her fury at being thwarted, XorX causes the cave entrance to collapse, trapping the four friends under the mountain. With their exit now blocked, they must find a way through the complex of mountain tunnels and a safe route back to the outside. On their journey, they discover the remnants of an advanced civilisation, encounter vicious creatures, and aggressive underfolk who resent them trespassing, and meet an apparently friendly robot who promises to help them. Their journey tests each of them to the limit, yet their individual ingenuity helps them survive a series of terrifying hazards as they try to outwit the underfolk pursuing them and find a path out of the mountain. Like the Ocellus story, Torus is vast in scope and intense in experience, and takes the reader on a rollercoaster ride of kidnapping, false hope, terror, grief and joy.