The Last Enchantment follows Merlin as he advises King Arthur during his reign. Merlin returns to the King after the cleaning of the shrine where Arthur claimed Macsen’s sword. King Arthur is going to follow up on the Saxon leader that got away from the battle at Luguvallium. He must prove his leadership. Cador is to escort Ygraine to Amesbury for Uther’s burial. Merlin is to escort King Uther’s body. He will also be there to present Arthur to his mother. Ygraine is also worried about meeting Arthur. Merlin can tell that she is ill, but says nothing. She and Arthur’s meeting is amicable. Merlin leaves them alone to become acquainted.
Merlin dreams of Morgause meeting King Lot in a house off a usually untraveled road. He can tell from their meeting that they have been lovers for some time. Merlin believes that she looks to Lot for marriage to obtain a crown and someone to claim the baby she is carrying.
Arthur makes his home at Caerleon. It is where he will be crowned. Ygraine sends a letter telling Arthur and Merlin that Lot and Morgause are married and they need not come to York for the wedding of Lot and Morgan. Morgan is happy at the news. Ygraine and Morgan opposed the marriage to Lot, but were conceding to Uther’s wishes. Arthur is angered by the union. He will accept it, since Morgan is not unhappy. She watches King Urbgen during the wedding feast and Arthur and Merlin see this as a good match.
Arthur assembles a cavalry that can move quickly to protect the Kingdom. Ygraine and Morgan attend Arthur’s crowning. King Lot attends giving his apologizes for his wife who is pregnant so could not make the journey. Arthur pales at this announcement. Ygraine is angered by the news. She believes Morgause seduced Lot and became pregnant so he would marry her.
Arthur knows that Morgause’s baby is his. He does not want the child to live and wants Merlin to devise a way to do it. Merlin refuses. He agrees to watch Morgause and then judge what to do. Arthur sends Ulfin with him to the North to spy on Morgause. They go in disguise. Arthur asks Merlin to assess two forts on his way North to see if they are worth fortifying in the East. It will be a slight detour on their journey.
The first fort that Merlin sees is in bad repair, but an excellent location. The second has been recently repaired. At Olicana, there is a sizeable township. The town is in impeccable order. Uther left a skeletal garrison there to watch the road into the Gap and keep an eye on the signal towers to the east. It was an emergency measure at the time and the men were never recalled so they stay per their orders.
The commander, Gereint, is a young man who Merlin believes will benefit Arthur. He keeps meticulous records of everything that happens in the area. He even has information on the Saxon’s movements. Gereint informs Merlin that Lot has a house in the area and that he and Morgause met there three or four times before their marriage and once for a month. It confirms Merlin’s suspicions that they were long-time lovers.
On the way North, Merlin and Ulfin acquire traveling companions. Beltane is a jeweler on his way to Lot’s court to sell jewelry to Morgause. Ninane is his servant. Merlin sees something special in Ninane. He believes he has the gift to be his successor. Merlin is upset when Ninane drowns while bathing in a river. His blames his lack of Sight, which has been nonexistent except for his dream of Morgause and Lot, since the morning in the chapel when Arthur took the sword.
Merlin finds Beltane a servant to replace Ninane. Casso has had his tongue cut out by his last master, Lord Aquisel, who has the tendency to cut out servant’s tongues so that they cannot tell of what they have seen. Merlin makes Beltane promise that he will teach the boy to write. Beltane gives them news of the baby, which is born too soon to be Lot’s. His name is Mordred. Lot is on his way from the King’s side to see his son.
Merlin finds a hiding place for he and Ulfin to watch for he believes that Morgause will send the baby away before Lot’s arrival and use a decoy baby. Lind, Morgause’s servant, comes rushing attempting to get to where Mordred is hidden for King Lot has killed the baby Morgause pawned off as Mordred and has ordered all the babies in the court killed. They find the woman to whom he was given dead in her home and no baby. Merlin and Ulfin chase after the soldiers, but are too late to help and the babies are set out upon the river in a boat to perish in the river.
Merlin does not believe that Mordred is dead. He stays on in the North to try and find any clue of where the boy is. Finding nothing after four months, he returns to Caerleon. Arthur has seen land where he would like to build a castle and asks Merlin to build it for him. The land is in Summers Country. The King is young and named Melwas. He is happy to have the High King build a stronghold at the edge of his land. It is to be called Camelot.
Arthur is to take a bride. She is a young woman whose parents have died and she is under the Queen’s protection. Her name is Guenever. He marries her at Caerleon. Queen Ygraine is too ill to make the trip for the wedding. Morgan is also to be married. Arthur asks Merlin to escort her to the court of King Urbgen for her wedding. Morgause attends Morgan’s wedding. Merlin is able to avoid her until the wedding feast where she is seated next to him. She tells Merlin that she does not know where Mordred is and asks Merlin to tell her if he knows. She is pregnant with Lot’s son. Merlin drinks too much wine and has to be helped to his bed. He spends the next day in his room and the following day sets out for Count Ector’s court with two escorts from King Urbgen. When they make camp that night, Merlin drinks from the wine he was given from Urbgen’s cellars. He remembers no more of this time. Merlin believes that Morgause poisoned him.
Merlin delves into madness and is lost in the forest. When he regains full alertness, he is with Arthur at Ector’s court. When they found him in the woods, he was talking nonsense and knew no one. It has been seven months since he wandered off into the forest. During this time, Ygraine died and Queen Guenever died miscarrying her baby. The Old Ones would send word when they would see Merlin, but he would then vanish. Merlin denies to Arthur that Morgause drugged him. Lot has died and Morgause is in Orkney with her children. Merlin tells Arthur that Morgause must live to rear her four sons by Lot because they will be Arthur’s faithful servants and the bravest of his companions.
Merlin recovers and journeys to Camelot. The work on it is progressing and ready for Arthur to move in. Arthur wants Merlin to stay at Camelot to advise him. He has missed him and was scared when he thought he was dead. Merlin agrees to stay, but builds his own home, Applegarth.
The nobles discuss Arthur needing a wife. Guinevere, a Welsh princess, is chosen for him. Bedwyr is sent with Gereint and King Melwas to escort her to Caerleon. Merlin does not meet her until she and Arthur travel to Camelot in early summer. She is a beautiful girl and admired by all.
One day when Arthur is away, Guinevere disappears. Bedwyr sends for Merlin. Merlin leads Bedwyr to where Melwas has taken Guinevere. Bedwyr saves her and takes her to Arthur who has just returned home. They meet him near Melwas’ castle. Due to Bedwyr’s injuries and Guinevere’s shaken disposition, they stay the night at Melwas’ castle rather than going home to Camelot. Merlin avoids Arthur and heads home to Applegarth for he believes that Guinevere went willingly with Melwas.
On his way home, Merlin believes he sees Ninane on a boat in the river. Only after inviting him to come to Applegarth to be his pupil, does he realize that the boy he saw could not be Ninane for if he was alive he would be much older.
The official story is that Melwas saved Guinevere when she was thrown from her horse. Guinevere told Arthur that she was kidnapped and held against her will, but nothing happened. Merlin is skeptical, but Arthur believes her. Arthur vows to get his revenge against Melwas someway. This comes when Melwas comes to the court to complain of harbor dues and taxes not being paid by the High King. Arthur steers Melwas to suggest single combat to settle the matter. Arthur wins the fight.
The boy Merlin spoke to on the lake comes to see Merlin. He has him call him Ninane. Merlin sees the hand of the god directing him to the boy and agrees to take him on as his pupil. He takes quickly to medicine and healing. Merlin enjoys teaching him. It makes him feel young again. Ninane also has the Sight. Merlin sends Arthur north on a vison of Ninane’s and it proves accurate. Ninane also sees the attraction between Bedwyr and Guinevere. When Arthur returns, he asks Merlin if he trusts Ninane. Merlin not only trusts him, he loves him. Arthur cannot believe how blind Merlin is and tells him that Ninane is a girl. The realization finally comes to him and he is not upset for he loves her.
Merlin tells Arthur that Bedwyr and Guinevere are in love. Arthur already knows and has known since the Melwas affair. He is glad she has someone to keep her company when he is away. He does not begrudge their love.
Arthur and Merlin confront Ninane. Her father was Dyonas, King of the River Islands. He sent her to the service of the Good Goddess. She could feel the magic in her blood and wished she was a man so that Merlin would teach her. When he spoke to her on the lake mistaking her for someone else, she had to take her chance. Her true name is Nimue.
Nimue and Merlin become lovers. He is proud of the power she has. He takes her on a pilgrimage of his past. With the end to be at Bryn Myrrdin where she can witness the Crystal Cave. Merlin falls ill on the journey. Believed dead, he is buried in Bryn Myrddin and the entrance sealed. He wakes to find himself entombed in his old home. He keeps himself alive on the food left by his funeral bed and on what he finds in the cave. Someone attempts to rob his cave and seeing Merlin alive runs in fear unfortunately for Merlin taking his rope with him so that Merlin cannot climb out of his tomb.
Stilicho comes checking on the tomb after hearing a story that a man came to Bryn Myrrdin and the enchanter came out of the tomb and spoke to him. Merlin wants his reappearance to life to be kept secret until he can tell the King. Stilicho tells him that Arthur has gone to Brittany. Together they manage to get Merlin out of the tomb and he recuperates at the mill with Stilicho and his family. He learns that Nimue has not only taken his place at Arthur’s side, but has become the Lady of the Lake. He sees his love for Nimue as a thing of the past.
Once he is recovered, Merlin prepares to travel to Northumbria to see friend, Blaise. On the journey north, he sees Morgause and her family traveling south. He hides so she does not see him. He wants to get word to someone that Morgause is headed south, but his journey must continue north because he cannot travel in the same ship with Morgause. Merlin goes to Macsen’s Tower to find the grail. He finds that a woman, a queen, has already found it and taken it away. Merlin travels south to warn Arthur that Morgause has the grail.
On his journey through Summers Country, Merlin happens upon a King’s Messenger. He asks the man to deliver a message privately to the King. The man is to tell Arthur that he met an old man on the road on his way to Camelot to see the King, but his way is slow so if the King wishes to see him he must come to him. He wraps his Dragon brooch in linen and asks the man to give it to the King. Seeing the brooch, the man knows that his is Merlin. The messenger tells him that the King will be quite happy.
Merlin learns from the messenger that Morgause is on her way to Camelot. She was summoned by the King. Arthur meets her at Camelot’s gate. His plan is to take her sons to live with him in Camelot and to murder her because he blames her for Merlin’s death. The messenger, Perseus, arrives while Arthur is speaking with Morgause. Perseus gives his message privately to the King. Arthur immediately rides out with Bedwyr and twenty of his knights.
Merlin goes out the next morning to meet Arthur upon the road. His is set upon by thieves. Arthur comes riding up alone and kills the three men. Only when the fighting is over does he realize that Merlin is their victim. They go to the inn where Merlin is staying to clean up, get refreshments and to catch up.
Merlin and Arthur ride to Caerleon. Nimue comes to Merlin there. She blames herself for not knowing he was alive. He learns that Nimue is the one who has the grail. She has married King Pelleas. Guinevere looks after Morgause’s children as if they were her own. Arthur has Nimue take the grail and hide it. It is meant not for Arthur, but for the future. Merlin returns to Bryn Myrrdin. He finds his cave and the people of the village a comfort to him. Arthur visits him often as a son would a father.