Loosely adapted from the legendary tales of Camelot, the plot adds the antagonistic Queen Mab and expands Merlin's backstory before the birth of King Arthur.

Part 1 Merlin, an elderly man telling his life story, describes a Britain faced with invasions and tyrant kings increasingly cruel to the people.

Years later, a teenage Merlin recounts to Ambrosia how he rescued a nobleman's daughter, Nimue, from drowning by magically growing a branch.

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.

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.

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.

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.

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.

Merlin meets a boy, Galahad, and his parents, Lancelot, a skilled rider and swordsman, and Elaine of Corbenic.

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.

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.

On the battlefield, Mab cannot save Mordred, and Arthur lives long enough to tell Merlin to return Excalibur to where it was found.

Mab slowly fades into nothing as Merlin, Frik, and the entire court of Camelot turn their backs on her and walk away.

[citation needed] Despite heavy fantasy elements, the production is partly historically accurate, keeping with the probable origins of the Arthurian legend in Sub-Roman Britain.

However, some anachronisms are present, like the use of the terms "knight" and "Sir" and the presence of Roman lorica segmentata armour years after it was discontinued.