When King Uther Pendragon sets his sights on Gorlois' lovely wife Igraine, little does he realise the chain of events he will set in motion. The resulting battle sees Uther claim Igraine for his wife and fathers a son with her who will one day become a legendary ruler. But Gorlois' young daughter Morgan vows revenge on her father's murderer and enlists an unwitting Merlin to help her develop magical powers.

Many years have passed since Arthur discovered his true inheritance and, after much fighting, most of England now accept him as their new King. But one knight, Accolon, refuses to recognise him as monarch due to the deaths of his brothers at Arthur's hand and vows revenge. And he is not the only one who seeks to harm him, but Arthur knows little of this when he encounters his half sister Morgan le Fey in the woods for the first time, unaware of her true nature. Meanwhile, Arthur becomes transfixed with Guinevere, the daughter of Leodograunce, but little realises what comes when he sends Lancelot to fetch her to attend his court in view of potentially marrying her.

With Merlin now departed, Morgan arrives in Camelot with her nephews Agravain and Mordred to offer her services to the King. Meanwhile, Sir Lancelot has been avoiding Guinevere due to his feelings for her, but finds an opportunity to distance himself from her when old King Pelles comes to ask for their help. He has been usurped by his nephew, while his daughter Elaine has been bewitched and kept prisoner in a tower guarded by a serpent.

A new knight offers his services to King Arthur in the shape of Guido de la Porte, but little realises the role he will play in events to come. As Morgan secretly plots her revenge on her half-brother, Agravain notices the close relationship between Lancelot and Guinevere, and spreads a rumour among the other knight. Meanwhile, Arthur decides to hold a tournament which Lancelot enters in secret, taking refuge in a friend's cottage to put on his disguise and in the process meets his daughter Eleanor, who proves helpful when events take a sinister turn.

Lancelot is being nursed by a hermit and Eleanor, who has fallen in love with him, after he is shot with a poisoned arrow. Arthur and Gawain seek out Vavasour's cottage that Gawain discovered earlier, only to encounter Morgan, who informs Arthur of Lancelot and Guinevere. When he returns to court, he finds Guinevere on trial, accused of killing Guido with a poisoned peach.

Years have passed and Camelot is seemingly at peace once more. An old man called Naciens arrives with a young charge named Galahad to offer his services to the Round Table. Soon after, a vision of the Holy Grail appears, leading to Arthur dispatching Gawain, Lancelot, Bors and Galahad to search for it. But while they are away, little does Arthur suspect that Morgan is plotting her revenge.

Guinevere finds herself on trial after Mordred accuses her of adultery with Lancelot, leading for him to call for her to be burned at the stake. Bors resigns in protest and joins forces with an escaped Lancelot to rescue Guinevere from her funeral pyre. Unfortunately, the rescue takes two knights as casualties, leading to Mordred to persuade Arthur to seek revenge on her with an army to France, where they are hiding out.

With Guinevere home in Camelot, Mordred claims that Arthur and Lancelot have been killed in battle in a message from France and declares himself King. But Bedivere is distrustful and decides to smuggle Guinevere out from Camelot and warn Arthur of Mordred's treachery. However, Mordred's alliance with the Saxon King Arcon only serves to alienate him from his aunt Morgan, who refuses to help when Arthur returns home, resulting in the ultimate battle for Camelot.