The Hollow Hills is Mary Stewart's second book in her trilogy of the era of King Arthur. It begins with the birth of a little boy, and ends with the beginning of a king. It is a book filled with power and the upbringing of a King. .
The story starts off with the end of the first book The Crystal Cave. After Merlin brings both Uther and Ygraine together for a night at Cornwall, a young child is conceived. Once the night is over, all hell is let loose, and soon many are dead because of the tryst. Uther, afraid how others may view him, informs Merlin that he will not consider the child an heir, but a bastard.
Nine months later, Merlin is summoned to Tintagel by Ygraine to take the child away because she is worried some may try and kill the only heir to the throne of England. After Merlin receives the baby boy, he takes the baby, a wet-nurse, and his servant across country to create a false story of where the child will be brought up. The child is taken to Moravik, Merlin's own nurse when he was young, to live until he is at the age of four. Once he reaches the age, he will be taken up north to live with Sir Ector. In the meantime, while the baby is growing up, Merlin decides to travel to the places he has only learned about in books. .
During Arthur's youth, Merlin becomes a holy man in the area near Sir Ector's court. Merlin soon becomes Arthur's mentor and slowly, but unbeknownst to Arthur, begins to prepare him for his future as King. .