Plot
Part 1
Merlin, an elderly man telling his life story, describes a Britain faced with invasions and tyrant kings increasingly cruel to the people. Fairy Queen Mab seeks to reclaim her power by drawing people back to worship The Old Ways. To do so, she creates a wizard named Merlin, whom she intends to champion her crusade. Merlin's mother dies giving birth to him, and he is raised by his guardian, Ambrosia.
Years later, a teenage Merlin recounts to Ambrosia how he rescued a nobleman's daughter, Nimue, from drowning by magically growing a branch. Ambrosia tells him of his past, and he is sent to Mab to begin training with Frik, Mab's gnome servant, however, Merlin has little interest in helping Mab's crusade to restore the Old Ways, angering her. Merlin discovers from the Lady of the Lake, Mab's sister, that Mab let his birth mother die and that Ambrosia is ill. Mab reaches Ambrosia's home before Merlin, but when Ambrosia refuses to convince Merlin to return to Mab, Mab lashes out, mortally injuring Ambrosia. Merlin tries to attack Mab but can't defeat her, she coldly dismisses the deaths of his mother and Ambrosia as "casualties of war" and vows that Merlin will help her. Merlin makes a blood oath to use his powers only to defeat Mab.
Many years pass. The tyrant King Vortigern is angered that his new castle keeps collapsing. Mab convinces Vortigern his castle will only stand if Merlin's blood is mixed with the mortar. Vortigern orders Merlin killed but relents when Merlin tells Vortigern of his vision: A crimson dragon representing Uther Pendragon defeating the white dragon that represents Vortigern. Mab tells the king that he can vanquish Uther by sacrificing Nimue to a dragon in the hope that Merlin will break his vow not to use magic. Merlin uses magic to save Nimue, and she is sent to Avalon to recover from her burns. Merlin asks the Lady of the Lake for help, and she gives him the magical sword Excalibur.
Merlin warns Uther of Vortigern's intention to attack him under cover of winter. Uther heeds Merlin's warning, and Merlin uses Excalibur to defeat Vortigern during battle, giving Excalibur to Uther afterward.
Uther becomes obsessed with Igraine, the wife of Gorlois, Lord of Cornwall. Seeing his emerging madness, Merlin tricks Uther out of Excalibur and plunges it into the Rock of Ages, who promises to keep it until a good man can withdraw it. Uther's mind is plagued by madness and lust for Igraine, and Britain slides back into civil war. Merlin strikes a deal with Uther to let him bed Igraine in exchange for guardianship of the son born from the union and for Gorlois and his men to be spared. Merlin transforms Uther's appearance into that of Gorlois, fooling everyone in Tintagel Castle except for Igraine's young daughter, Morgan le Fay. After bedding Igraine, Uther has Gorlois and his men slaughtered, deliberately going back on his word to Merlin.
While Igraine is in labor, at Mab's behest, Frik convinces Morgan to place a black stone beneath the baby's sheets in his crib. Igraine gives birth to Arthur. Mab then confronts Merlin, proclaiming Arthur to be damned, but Merlin vows to raise him in the ways of good and hopes Arthur will help bring about her demise.
Part 2
Merlin begins tutoring Arthur. Meanwhile, Uther sinks further into madness and commits suicide, leaving the kingdom in turmoil. The noblemen in the kingdom try to take Excalibur from the Rock of Ages, but none can until Merlin presents Arthur, who is allowed to take the sword.
After initial hostility, the nobles unite behind Arthur as their rightful king. Merlin leaves, thinking the kingdom is finally at peace. Mab, however, instructs Frik to seek out Morgan and poison her mind by making her beautiful and having her seduce Arthur, who is unaware of their half-sibling relationship. Merlin races back to Camelot to confront Arthur and Morgan gives birth to Mordred, who, conceived through incest, is born evil. Mab helps raise Mordred to become Arthur's downfall. Arthur starts construction of his castle of Camelot and marries Guinevere.
Arthur decides to take his knights on a quest for the Holy Grail, he holds a tournament to crown a champion who will defend and complete Camelot in his absence. The Lady of the Lake vows to guide Merlin to a man worthy of being Camelot's guardian. Merlin meets a boy, Galahad, and his parents, Lancelot, a skilled rider and swordsman, and Elaine of Astolat. Merlin brings Lancelot to Camelot, where he wins the tournament.
In Arthur's absence, Guinevere and Lancelot embark on a love affair. Mab makes sure Elaine sees his betrayal, and she is found dead at the shores of Camelot not long after. Lancelot flees Camelot in his guilt and shame. Arthur and his knights return, their quest for the Grail a failure.
Mordred introduces himself as Arthur's son and heir and reveals Guinevere's betrayal to all. Arthur is forced to condemn her to be burned at the stake for treason, but he relents and has Merlin save her before she is harmed, causing him to lose respect among the younger nobles. Lancelot rides back to Camelot to save Guinevere, and they ride away into exile.
Mordred raises an army among those dissatisfied with Arthur and Merlin. Meanwhile, Frik and Morgan have fallen in love. When Morgan refuses to allow Mab to use Mordred further, Mab kills her. Frik vows revenge on Mab but is left powerless when she takes away his magic.
Mab creates an idyllic wilderness home for Nimue and asks her to persuade Merlin to stay with her, hoping to stop him from intervening in the coming battle. Wanting to be with him, Nimue agrees and sends for Merlin.
The armies of Mordred and Arthur begin the Battle of Camlann, where many on both sides are slain. Arthur defeats Mordred and kills him, but Mordred deals Arthur a fatal blow. Sensing his protégé is dying, Merlin leaves Nimue to go to Arthur, but Mab's creation seals itself behind him, parting Merlin from Nimue forever. On the battlefield, Mab cannot save Mordred, and Arthur lives long enough to tell Merlin to return Excalibur to where it was found. Merlin gives back the sword to the Lady of the Lake. She tells Merlin she is slowly dying as the Old Ways are forgotten. Merlin accuses her of lying to him about the guardian of Camelot, but she explains that it was Galahad who was the true guardian and could have averted all this, though she assures Merlin that Arthur's death is not his fault. Merlin encounters Frik among the survivors of Arthur's army, who warns him that although Mab is significantly weakened, she is still dangerous.
Merlin confronts Mab at Camelot, but the ensuing magical duel ends in a stalemate. Mab sneers that she is invulnerable to conventional means of destruction, but Merlin responds that she will be defeated when she is forgotten. Mab slowly fades into nothing as Merlin, Frik, and the entire court of Camelot turn their backs on her and walk away.
Merlin is again shown back in the present, finishing his story. Frik reminds Merlin that he remembers things differently. Merlin admits that what Frik said is true however he could not tell that story to his listeners. Frik leads Merlin to Merlin's aged magic horse, who Frik says will take him to Nimue. Frik explains that sometime after Mab disappeared, the spells she had cast lost their effectiveness, and Nimue was set free. Merlin finds the elderly Nimue at Ambrosia's old forest home. He manages one last act of magic to restore them both to youth so they can finally live out their lives together.
