The Hollow Hills follows Merlin as he guards the child of Uther and Ygraine from the King’s enemies. Merlin must use his persuasion to get the King to let him do what is in the best interest of the child. The baby is handed over to Merlin just three days after his birth. Merlin and Ralf along with the baby’s wet nurse go to Brittany to a remote inn where Merlin’s childhood nurse will help care for the child.
Merlin leaves Ralf to protect the child and travels Europe to throw off the spies that he knows will look to him for the whereabouts of the child. When Arthur is three-year-old, Ralf takes him back to Britain to be reared in the court of King Ector as the man’s foster child. Merlin made the arrangement prior to traveling to Brittany. Merlin watches over Arthur through his Sight, while he travels. Hearing that the King is ill and that the Queen has delivered a stillborn son, Merlin travels back to Britain. He goes to see the King to find him very ill and contemplating bringing his son to court. Merlin tends to the King’s health and convinces him that Arthur’s best where he is for now. Uther’s illegitimate daughter, Morgause, is left in charge of tending to the king. Morgause wants to learn more of the craft from Merlin, but he does not plan to stay at the court.
Merlin returns to Maridunum with his servant. He must stay there a time to keep from leading spies to Arthur. Merlin waits there until the spies show themselves. He finds men around his cave when he returns home. The men seem surprised to see him and try to make Merlin think that they are sent by the King. Merlin knows from their speech that they are Cornish soldiers. He pretends to believe them and gives them a letter for the King, which he writes in Greek so that they will not be able to translate it quickly and it will give him time to leave Maridunum.
Merlin travels north through the rugged country avoiding roads. He cannot afford to be recognized. During his travels, he is accosted by the people of the hollow hills who have been offered money to follow a man with Merlin’s description. When they discover that he is Merlin, they vow their protection of him and his secret. They also vow to keep an eye on the young boy who will be the King of all Britain and will unite all people.
Merlin is lead to Macsen’s palace and finds Macsen’s sword, which has been haunting his dreams. He knows that this sword is for Arthur. A blacksmith tells him of a holy man who knows the story of the sword and Merlin searches him out even after he has found the sword. He finds the man injured and dying. He does his best to care for him. The priest tells Merlin how the men who returned to Britain passed by this shrine and carved the likeness of Macsen’s sword into the alter before traveling on to his home to store their treasure. Each successor of the shrine, is told the story of the sword and how it will one day return to the alter so the priests have kept the shrine prepared for that day.
Merlin takes over the duties of caring for the shrine when the priest dies. He sees it as an unobtrusive way to keep an eye on Arthur and he can have Count Ector send Arthur to him for lessons without arousing suspicions. Merlin hides Macsen’s sword on the island in the lake that he passes on his way to the village for supplies. Caer Bannog is said to be haunted and no one goes there because men fear it. He believes it is the perfect hiding place.
Merlin learns from Count Ector that Lot, who is betrothed to Princess Morgain, is actively seeking Arthur. He has sights on the throne of the High King. He may also side with the Saxons when the war comes. It would have to be after his Christmas wedding to Morgain to make him a legitimate heir to the throne.
Arthur and Merlin become fast friends. Ralf is relieved to see him because the boy is a handful. Merlin teaches Arthur and his friend, Bedwyr who has come to take lessons with Marcellus, the best swordsmen in the country. The King had him sent to Ector’s court when he learned Arthur was staying there. The boys learn military tactics and medical skills with Merlin as well as history. When Bedwyr returns to his own court, Arthur looks more to Merlin for companionship. One day he comes bursting through the woods chasing a white stag, while Merlin is fishing at the lake. The stag swims the lake to Caer Bannog. Arthur’s dog, Cabal, follows and Arthur goes after him.
While Merlin waits for Arthur to return, Cador of Cornwall and his men appear. Cador is riding north to war. He tells Merlin that men have been looking for him everywhere. When Arthur appears rowing across the lake, Cador is enthralled by the sword he carries. Arthur attempts to protect Merlin, but he tells him that Cador is an old friend and he has no need of protection. Ralf takes Arthur back to the Green Chapel for safety.
Cador knows that the boy is Uther’s son. He can see the resemblance between he and Merlin. He tells Merlin that he is not the boy’s enemy. He wants to protect him so that there will be someone to contest Lot’s bid for the throne. The war is coming sooner than Lot would have wished. Cador says that he will have to choose between the Saxons and King Uther. If the Saxons had waited until he was married to Morgain, he could have used the Saxons to take the throne. The King is on his way to Luguvallium to meet the coming Saxons and Cador is on his way to meet him there. Uther is sick and Cador believes that he will send for Arthur. Merlin proclaims that Arthur will be there whether he is summoned or not with Merlin by his side.
Merlin tells Arthur that the sword that he found is the sword of Macsen Wledig. Arthur questions Merlin about who he is after hearing Cador call him an enchanter and seeing one of Merlin’s visions. Merlin confides in him that he is Merlinus Ambrosius, son of Ambrosius, the High King. Arthur tries to give him the sword saying that it belongs to his family. Merlin tells him that the sword is for the legitimate heir to the kingdom. The King’s men arrive to escort Merlin to the King. He informs Arthur that he will be riding with him. Merlin has Arthur leave the sword in the chapel. He uses his magic to make the sword part of the stone altar.
By the time Merlin and Arthur reach Lugvallium, the King’s forces are already moving into position for attack. They are taken directly to the King’s tent. Merlin is surprised by Uther’s appearance. He looks very ill and will be carried in a litter to the battlefield. He looks at Arthur with pride. Merlin thinks that he will proclaim Arthur his heir right them, but the battle horn sounds and everyone must take their positions. Arthur is given a sword by Count Ector, but the King orders him to fight beside him. Arthur is dressed all in white and riding a white horse for he has no family crest or colors because he is illegitimate and does not know who his father is. This white ensemble makes him stand out from the crowd.
During the battle, Lot holds back in the woods in an apparent tactic to rush in when the King needs fresh troops, but Merlin suspects it is to see how the battle goes to see which side he will choose. He times his rush too late and Arthur takes the day. Arthur saves the King from death, but loses his sword in the process. The King readily throws his sword to him and Arthur fights back the Saxons as the King sends his banner with him so that the men follow Arthur and win the battle.
Merlin is surprised to find Morgause came with the King. Arthur blushes at her beauty and stammers a greeting. Merlin tells him to stay in the room, while he goes to see the King. The King plans to proclaim Arthur his heir at the victory feast the following day. Merlin feels a darkness as he returns to his room, but does not know why. He finds that Arthur is not there, but has gone off with a young woman. The guards felt it was his right after proving himself in battle. She was one of Morgause’s girls so they thought he would be safe. Merlin uses his power to discover that Arthur did not sleep with Morgause’s girl, but with Morgause.
Merlin confronts Arthur when he returns and tells him that he is the King’s son. Arthur is a bit shaken for he thought that Merlin was his father, since they looked so much alike. When he realizes that Morgause is his half-sister, he is disgusted. Merlin tells him that he is innocent because he did not know she was his sister. He tells him to put it behind him and that he will deal with Morgause. Arthur must sleep and prepare to see the King.
Merlin sends for Morgause and can feel the evil in her. He knows that she is with child and that she planned it. He tells her to leave and go to Morgain to be with her before the wedding that she will not be wanted at court. The King will not want to see her. Merlin knows that Uther will die this day and that Arthur will be King. Morgause is shocked by this. She tries to defy Merlin, but her power is too weak against his. She leaves as he requests.
Uther proclaims Arthur his heir at the victory banquet. Lot protests that they cannot follow a boy who has only been in one battle. Lot proclaims that Uther should give Arthur his sword now and then the men would have to acknowledge him as heir. When the sword is taken from the scabbard, it is broken. This causes alarm among the men assembled. The kingdom cannot be protected with a broken sword. It is an omen that Arthur is not the right choice. After seeing his broken sword, the King dies. Merlin notices that Lot does not seem surprised by the broken sword and knows he is behind it being broken. Cador proclaims that Arthur has a sword to defend the Kingdom and it is better than any other sword for it is the sword of Macsen, which proves him the true heir.
Merlin stands behind Arthur for he is taking his place as Arthur’s servant. He tells the crowd that Arthur was given a sword straight from God. Arthur will lift the sword from stone in front of all these men at the Green Chapel, which from now on will be called Chapel Perilous and any man who is not the rightful King will be burned, if they attempt to touch the sword. Arthur speaks to the men assembled with authority after Merlin’s speech and says that in the morning they will all go the Chapel and he will take up the sword in front of them all.
Merlin leaves the camp to return to the Chapel. He knows that Lot will attempt something and he does not want the priest who is looking after the Chapel to be hurt. He is accosted by two of Lot’s men and must fight them off. He loses consciousness and wakes to find the people of the hollow hills caring for him. He has them clear the road that the men obstructed so that the King and his men can get to the Chapel in the morning. The men readily do so because Merlin has promised them that Arthur will not forget them as other kings have.
Merlin prepares the Chapel for the morning. When the men arrive, the altar is lit by flame it runs down the length of the sword making it white hot. He tells the men to take up the sword he who dares. No one wants to attempt it. Arthur comes forward and puts his hand through the flames and picks up the sword. Lot is the first to kneel to Arthur. Arthur raises him up and tells him that he has no quarrel with him. He proclaims Cador his heir until he has an heir of his own. Each man takes an oath and pledges himself to the King. The men joyously accept their new King.