The Hollow Hills, written in 1970 and published in 1973, is the second in a quintet of novels covering the Arthurian legends by Mary Stewart.
The novel follows Merlin, the protagonist and narrator, who supervises the birth and raising of King Arthur. The novel begins the day after Arthur is conceived. After being thrown from his horse and knocked unconscious, Merlin awakes to find that Duke Gorlois of Tintagel has been killed in a fight, which occurred because King Uther Pendragon wanted to sleep with Duchess Ygraine of Tintagel. Using Merlin and his magic, Uther slipped past the guards and slept with Ygraine. Merlin, with his ability to see into the future, knows that during this union, the next great king of England was conceived.
Ralf, a young boy, brings Merlin word of Ygraine’s pregnancy, just as he predicted. Merlin convinces Uther and Ygraine that the best thing for the baby would be to have Merlin raise him in secret, to keep him safe from Uther’s enemies.
Months later, Merlin is called to Tintagel for the birth of Ygraine’s son, Arthur. The baby is handed over to Merlin at just three days old. Merlin realizes that Uther’s enemies will assume that the baby has been given to Merlin, so he leaves Arthur in the care of the woman who raised him, and tasks Ralf to stand guard to watch for danger.
Merlin leaves and travels east to Constantinople. He has visions of a sword that belonged to Magnus Maximus, the first king of England. Merlin realizes that the visions are leading him to the sword. However, he must return to England when he receives word that Uther has fallen ill, and has yet to produce another male heir, making Arthur his only male heir.
When Merlin arrives at Uther’s bedside, he is questioned about Arthur’s whereabouts. Merlin tells Uther that Arthur is staying at Count Ector’s court and that he can be trusted to take good care of his son. He takes his leave, planning to head back to Arthur, but realizing that Uther’s enemies will be spying on him, he has to be careful not to lead them directly to the King’s sole male heir.
Along his journey, Merlin is distracted, following a number of signs and visions that ultimately lead him to a tower that is now in ruins. After exploring the ruins, he finds a sword, the same one that has been haunting his dreams. He knows that this sword is for Arthur. He visits an old chapel sanctuary above Ector’s home, and he knows that this will be a special place for Arthur. Later, a young boy rides up on horseback to the altar and Merlin immediately knows he is Arthur. However, he does not explain to Arthur who he is, revealing himself only to Ralf and Ector.
Now nine years old, Arthur is fascinated with Merlin and the stories of his travels. Merlin teaches Arthur about the ways of kings, helping to develop his character so that he may one day take his rightful place on the throne. One day, a group of soldiers visits Merlin, who at first believes that they are on a mission to kill him and Arthur. He soon realizes this is not the case; they have come to recruit Arthur for battle. Merlin acknowledges that it is time that Arthur goes to fight at his father’s side.
When they arrive at the battle, Merlin spots Uther who looks to be near-death. He tells him that his son has arrived, but before they can make the proper introductions, the Saxons attack. Merlin watches from the sidelines as Uther, Arthur, and the other soldiers head into battle. The Saxons, a mighty army, almost defeat the Britons, nearly killing Uther in the process. Just as things are looking dire, Uther tosses his sword to Arthur who leads the troops to victory. Observing this, Merlin knows that this moment will secure Arthur as the true leader in the hearts and minds of the troops.
The group returns to celebrate at a victory festival, where Uther proclaims Arthur his long lost son and the heir to the Pendragon throne. Uther is challenged by Lot, a lesser king, who says that Arthur isn’t ready to assume such responsibility; Uther tries to hand over his own sword to his son, granting him the position of the throne immediately. However, his sword breaks and Uther immediately dies. Merlin recalls the sword he found in the temple ruins, mentioning that it is enchanted; only a true king can lift it. The group watches in awe as Arthur easily lifts the sword out of the stone, and he is hailed as the true king of England.